Chapter 11 Infectious Disease

Infectious Disease Control

Clinicians are key players in controlling the spread of infectious disease. They are typically the first to see new cases of a disease that could indicate the beginning of an epidemic. They must report these to public health authorities. At times, they are called upon to collaborate with infection control or with public health teams in order to ensure good control practices and to arrest outbreaks of infectious disease. Clinicians must also be aware that they themselves or the equipment they use can transmit infection to their patients and must take steps to prevent this.

Clinicians are also in a position to recognise changing patterns of non-infectious disease, for example an unusually high number of injuries that may be associated with a change in the built environment or with popular toys and pastimes. In these cases, clinicians can also play a role in identifying and managing the situation, although the process for doing so may not be as clearly defined as the process for infectious diseases.

After reading this chapter, you will be able to:

Linking these topics to the Medical Council exam objectives, especially section 78-5.

Dr. Rao suspects an outbreak

Dr. Rao received a phone call from Mrs. Richards’s long-term care home to say that she has had some diarrhoea. He went to see her. She says she had some stomach pain last night but feels better today; she just had one bout of fairly liquid stool. Mrs. Richards thinks that other people in the home have been sick also. She heard that one resident had vomited two days ago and another had stomach cramps the other night.

Dr. Rao tries to get more precise information; he wants to know if this number of people with stomach complaints is greater than normal. He also realizes he should ask a whole series of questions: precisely who was sick, when, for how long, the location of the rooms of the sick people, if any of the staff have been sick, and if the Public Health department has been informed. However, the staff member now on duty was away for the last few days and knows only what was mentioned in the daily report.

Detection and Control of Outbreaks

Clinicians are often in the best position to recognize an OUTBREAK of disease (see Definitions: Outbreak terms); sometimes they see an unusually high number of people with the same disease and at other times patients report that they know other people with similar symptoms. Even if the clinician only sees a single case, it may be one of a number in the region so reporting contributes to the detection of outbreaks. Filling in discharge summaries and death certificates, and reporting cases of notifiable disease (see: Notifiable disease) all contribute to health surveillance. Ensuring that the forms are completed accurately and transferred in a timely fashion makes a major contribution to outbreak detection and general health surveillance.

Depending on the type of organism involved, the conditions of spread and the target population, disease outbreaks can be acute and fast-moving, such as gastroenteritis in a nursery school or long-term care home, or they can evolve more slowly, such as the AIDS pandemic. While public health authorities are ultimately responsible for ensuring the detection and control of outbreaks, clinicians are major players because they are usually the first point of contact with the affected population. Likewise, hospital infection control teams rely on the cooperation of clinicians in preventing infections. The basic steps in outbreak management and control are:

  1. Establish the existence of an outbreak
  2. Define what constitutes a case and identify cases as they occur
  3. Formulate hypotheses on the causes, and implement initial control measures
  4. Test the hypotheses through analysis of surveillance data or special studies
  5. Draw conclusions and re-adjust hypotheses and control measures if needed
  6. Plan for long-term prevention and control.

Stage 1: Establishing the existence of an outbreak

Is it an outbreak?

It is important to identify an outbreak quickly so that it can be addressed before it develops into a full EPIDEMIC. However, there is no hard-and-fast rule for defining an outbreak. A cluster of cases could be due to chance or could form the beginning of an outbreak. This is especially true of rare diseases and diseases in small populations, where an absolute increase of a very small number of cases could represent a large relative increase in an illness in the population. For instance, in a community where there are usually two cases of a given disease per month, four cases in one month (relative increase of 100%, but absolute increase of 2 cases) may or may not constitute an outbreak. In such instances, it can be very difficult to decide how much time and money to spend on investigating something that might be just a chance occurrence. Analysing the complete history of each case helps, but consultation with epidemiologists or statisticians may be required.

Furthermore, biases may occur in documenting an outbreak. If a possible outbreak has been discussed on the news, and if clinicians are aware of a possible outbreak, they may be more assiduous than usual about requesting laboratory tests and reporting cases. If patients are aware, they may be more sensitive than usual to their symptoms and may seek care (and hence their case gets reported) when they would not normally do so. In addition, case-finding or active surveillance (see Chapter 7), by public health workers may uncover cases that would not otherwise have been identified. 

Outbreak terms

Endemic is the constant presence of a disease or infectious agent within a given area or population group.1

Cluster is a collection of new cases of an uncommon disease that occur so closely together in space or time as to arouse suspicion that this is not merely a chance occurrence.1

Outbreak is two or more cases of illness thought to be linked in time and place.2 An outbreak can also refer to a small, localized cluster of cases, usually of an infectious disease that may be a prelude to a broader epidemic.1

Epidemic is the occurrence of illness clearly in excess of normal expectancy.1 The number of new cases needed to declare an epidemic depends on the disease, the time, and the location. For example, a single case of yellow fever could be considered to be an epidemic in Canada, but it might not be in a tropical region.

Pandemic is an epidemic occurring over much of the world. Examples include influenza (1918–19 and 1957–58), cholera (1961 to present), AIDS (1988 to present), and tobacco addiction (present).1

The distinction between cluster, outbreak and epidemic is fluid, but cluster suggests a narrower geographic distribution than epidemic, perhaps an early stage in the development of an outbreak or epidemic. Cluster applies to both communicable and non-communicable diseases.

Great care is needed in deciding whether or not to declare an epidemic: balancing the value of alerting agencies to the need for control measures against possible concerns over causing public panic. National and regional public health agencies are responsible for these decisions. A pandemic can only be declared after careful evaluation by the WHO, which may issue travel advisories, and such announcements are taken only after serious deliberation because of their potential impact on the economy of regions affected.

Just as a local outbreak runs a natural course (see figures 7.1 to 7.5  in Chapter 7), so does a pandemic. As an example, Figure 11.1 illustrates the stages of influenza outbreaks as described by the World Health Organization:

Figure 11.1: World Health Organization levels of pandemic (Source: )
Figure 11.1: World Health Organization levels of pandemic (Source: )

Infectious disease epidemics can result from:3
1. Increased virulence of the infecting organism. Micro-organisms have a number of mechanisms to alter their virulence. For example, Corynebacterium diphtheria must be infected with a specific bacteriophage in order to produce the diphtheria toxin that causes diphtheria. Plasmid exchange can confer antibiotic resistance in previously susceptible bacteria. Another example is the influenza virus, whose virulence may vary as its genetic composition drifts and (See glossary for definitions of genetic drift and shift and VIRULENCE)

2. Recent introduction of the organism into new setting. Europeans introduced measles and smallpox to the Americas, where they swiftly decimated the local populations who had no immunity to them. The two diseases were among the causes of the decline of the Aztec empire. More recently, the severity of the 2002-03 North American epidemic of West Nile virus may have been in part due to the lack of immunity of the host birds in the region, as well as the lack of immunity of the human population (see Nerd’s corner).

West Nile virus in North America

Figure 11.2a Patters of West Nile virus in USA, 1999-2008
Figure 11.2a Patterns of West Nile virus in USA, 1999-2008

Figure 11.2b Patters of West Nile virus in Canada, 1999-2008
Figure 11.2b Patterns of West Nile virus in Canada, 1999-2008

West Nile virus was first documented in Uganda in 1937. In the 1990s, in Europe, the virus seemed to increase in virulence. In North America, it was first detected in birds in New York, and in 2002 and 2003, it caused a large epidemic, more widespread than in other continents. Reasons for its epidemic potential in North America may include: 4

  • More virulent strain
  • No previous immunity in birds – the primary host
  • The carrier mosquito, Culex pipiens, mainly feeds on birds, but switches to feeding on humans in the fall when host birds migrate

Age structure of non-immune individuals—older and more susceptible to neurological effects.

3. Enhanced transmission so that more susceptible people are exposed. Troop movements and population upheavals during the 1914–1918 war brought many more people than usual into close contact with one another. This enhanced the transmission of the influenza virus that caused the 1918 pandemic.

4. Change in host susceptibility. People with HIV infection are particularly susceptible to tuberculosis. The current high incidence of tuberculosis world-wide is, in part, due to the HIV epidemic increasing the number of susceptible people.

5. New portals of entry or increased exposure. Technical developments in health care that require invasive instrumentation have contributed to the rise in nosocomial infections.

Walkerton, Ontario5

On May 18, 2000, a paediatrician at the Owen Sound hospital admitted a nine year-old girl with bloody diarrhea and a seven year-old boy with fever and abdominal pain. The boy later developed bloody diarrhea. On investigating possible links, the attending paediatrician found that the children attended the same school. The same day, people from Walkerton started contacting their Public Utilities Commission because they suspected problems with the water supply.

On May 19th, having learned that more people in the community were suffering from diarrheal illnesses, and having conducted a food history with the parents of her two patients, the paediatrician suspected that they were suffering from water-borne Escherichia coli infection. She contacted the Bruce-Grey-Owen Sound Health Unit. Later that day, the managing director of a Walkerton retirement home also contacted the health unit to report that three residents had diarrheal disease.

The health unit contacted schools in the area and learned that abnormally high numbers of pupils were absent with illness. The unit also learned that the Walkerton emergency room had seen eight patients with diarrheal disease. At first, the pattern of disease suggested a food-borne outbreak, but, as many members of the public believed the outbreak to be due to contaminated water, the health unit also began to investigate the water supply.

The Walkerton outbreak was caused by contamination of the municipal water supply with E. coli and Campylobacter jejuni. By August 31st, 2000, it had killed seven people, caused Haemolytic-Uraemic Syndrome in 27 people and, it was estimated, made over 2,000 people ill. Poor supervision of water treatment combined with heavy rain, which had washed cattle effluent into the water supplying the treatment plant, were the major causes of the outbreak.

Factors contributing to the Ebola outbreaks

In 2013 and 2016 in West Africa there were outbreaks of Ebola virus that were worse than previous outbreaks. By April 2016, 11,310 people had died6. Before that the largest Ebola outbreak was in Uganda and killed 224 people. Of the factors that contributed to the severity of 2013 – 2014 outbreak some were cultural and socio-economic:

  • The outbreak started among impoverished, marginalized rural population. These people are forced to exploit the forest as a source of food and resources. Going into the forest puts them at greater risk of being infected by forest animals that carry the disease.
  • The region had been decimated by years of civil conflict which left the economy and the public health system unable to respond adequately and early in the outbreak.
  • Porous borders between the 3 countries affected meant that a high level on intergovernmental cooperation was required
  • Poor healthcare infrastructure in the affected areas meant that patients were being cared for in facilities that had inadequate means of diagnosing infection and preventing transmission.
  • The traditional rites of bathing and touching the bodies of relatives ensured spread to family members. Mistrust of health advice made it difficult to address these practices

For further information see the article by Bausch and Schwarz7. For a view from further upstream, see: Richardson et al.8

Notifiable (or reportable) disease

Physicians are required by law to report cases of certain infectious diseases. Unfortunately, many do not. Reporting disease is necessary in order to direct attention to possible outbreaks or single cases that will require rapid action to control. The notifiable disease database can also be used to evaluate resulting control measures, as well as to trace long-term fluctuations in incidence, indicating changing patterns of behaviour that may require intervention on a number of levels to control the disease.

In Canada, maintaining the notifiable disease information system is a provincial and territorial responsibility. The World Health Organization specifies a number of diseases that must be reported worldwide. The Public Health Agency of Canada specifies some that are notifiable nation-wide and each province or territory can add other diseases to be reported in that province or territory. Not all infectious diseases are notifiable. The Public Health Agency of Canada selects diseases as notifiable according to certain characteristics, such as:9

  • Its interest to national or international regulations or prevention programmes
  • Its national incidence
  • Its severity
  • Its communicability
  • Its potential to cause outbreaks
  • The socio-economic costs of its cases
  • Its preventability
  • The risk it poses in the public perception
  • The need for a public health response
  • Evidence that its pattern is changing.

Some provincial and territorial public health authorities require physicians to notify them when they suspect an outbreak of any infectious disease. Some lists include non-infectious diseases that can be caused by environmental hazards, such as poisoning with heavy metals or with carbon monoxide.

Physicians are required to notify the public health authority when history and clinical examination causes them to suspect a notifiable disease; they are not required to await the results of testing. Indeed, diseases that could pose an immediate, severe threat to the public’s health should be reported by telephone as soon a case is suspected. Laboratories notify the public health authority of cases of notifiable disease when test results are positive. Because some diseases are difficult to diagnose reliably without laboratory testing, while others do not require testing, the list of laboratory-notifiable disease differs slightly from that of physician-notifiable disease.

Stage 2: Define and identify cases

A crucial early step in investigating a possible outbreak is to define what constitutes a case, as the case definition will be used in the search for more cases in order to draw a complete portrait of the outbreak (see Definitions: Defining a case). The case definition describes precisely the symptoms, signs, history, or test results that indicate a probable case of disease. Clinicians may be asked to help in the search to uncover all cases, and they may also have valuable information linking the cases, which can suggest a possible cause.

Because of biological variation, the presenting symptoms and signs for cases of any illness vary. The case definition should neither be too broad nor too narrow. In an outbreak of gastro-enteritis, for instance, some people may only have mild abdominal cramps, whereas others have diarrhoea and vomiting, with or without fever, muscle pain, headache, and dehydration, etc. If the case definition includes people with any one of the full range of symptoms (fever, or muscle pain, or headache, etc.), it will be broad enough to include a many people whose symptoms are unrelated to the outbreak under investigation (a sensitive test). Conversely, if too narrow a case definition is used (fever and muscle pain, and headache), it may exclude many people with the disease, hence underestimating the extent of the outbreak and possibly delaying the implementation of extended control measures (a specific test) (see Chapter 6). If there are indications of a common exposure, for instance if a large proportion of the initial cases say they ate at the same restaurant, the common exposure can be included as a criterion in the case definition to specify the cases involved and the investigation could focus on identifying the particular foodstuff at the root of the problem.

Defining a case

Case definition is a set of criteria that must be fulfilled in order to identify a person representing a case of a particular disease as part of an outbreak. These criteria may include geographic, clinical, or laboratory criteria, and may be combined into a scoring system. The case definition is used to identify probable cases in epidemiological surveillance.1

Note that the objectives of using criteria to identify cases in the context of infectious disease control are entirely different from using criteria to identify cases in a clinical, diagnostic situation. In infectious disease control, cases are identified so that they may be fully described and so that common factors may point to the source of the problem. In clinical situations, the use of criteria helps identify patients who are likely to benefit from treatment.

Stage 3: Formulate hypotheses and implement initial control measures

Using the information gathered in the initial steps of the investigation, public health professionals describe the epidemic in time, place, and person. As the outbreak progresses, they may draw an epidemic curve to track its evolution in time (See epidemic curves in Chapter 7). They describe in detail the circumstances of the outbreak and the demographic characteristics of the people affected. The patterns found usually indicate the likely source of the outbreak and the population at high risk. This is generally sufficient to suggest some initial control measures.

Management of cases

In general, cases in an outbreak are managed by their usual physicians unless the size of the outbreak justifies setting up special clinics. Management includes the prevention of transmission of the disease by isolation of the patient, providing prophylaxis and immunization of the contacts where possible and education on hygiene such as hand washing and respiratory hygiene. The clinician can consult the public health service for advice on the control of spread. In the case of sexually transmitted infections, some public health services trace and treat the patient’s sexual contacts. In outbreak situations, public health officials keep health professionals and the public informed of the situation and of the control procedures.

Management of outbreaks

The public health service is responsible for managing outbreaks in the community. Clinicians may be called upon to collaborate in this. People at risk from the outbreak—that is, those exposed or likely to be exposed to the probable source—are identified. The people at risk need:

  • Information about their risk and how to reduce it. For instance, forestry workers should use adequate clothing and know the symptoms of Lyme Disease so they seek treatment early
  • Personal preventive measures. For instance, close contacts of meningococcal meningitis should have antibiotic prophylaxis; health care workers may need HIV prophylaxis after a needle-stick injury; contacts of people with a disease for which there is a vaccine could be immunized; sexual partners of people with sexually transmitted infections may require treatment
  • To reduce the risk of propagation. For instance, this can be done by putting asymptomatic contacts into quarantine for the duration of the disease’s incubation period, or by asking people to “cough into your sleeve” during the flu season. Adequate treatment of the infection also reduces transmission.

Dr. Rao reminds the staff…

As he was at the nursing home, Dr. Rao took the opportunity to remind the staff about hand-washing. He also reminded the manager that the nursing home staff should use gowns and gloves and disinfectant solution for cleaning up any “accidents.”

Mumps in the Maritimes

In Halifax, Nova Scotia, in 2006 and 2007, two successive outbreaks of mumps affected a total of 34 people.10 Although there was no obvious link between the outbreaks, they were thought to be part of a group of outbreaks that occurred in North-Eastern USA and Québec, and which may have originated in the UK. The Nova Scotia outbreaks were managed by voluntary isolation of the cases for nine days after symptoms appeared. Households and other contacts of the cases were immunized. All cases had been immunized against mumps, although many had received only one dose of vaccine. It was decided not to immunize all those at risk of the illness because:

  • Mumps immunization levels were already high
  • Spread of the outbreak was limited and slow, and there was no evidence of spread to unimmunized groups
  • It was assumed that herd immunity was effective because of high immunization levels and the limited spread of the disease (See Glossary HERD IMMUNITY)
  • Experience with a similar outbreak in the UK showed vaccination had a poor cost-benefit ratio
  • The population at risk was large.

Stage 4: Test hypotheses through analysis of surveillance data or special studies

Once there is a hypothesis about the cause of the outbreak, it should be tested. If removing the suspected source is followed by a decline in the outbreak, the hypothesis may be correct, although the decline could still be serendipitous. Cohort studies can be carried out in the case of food-borne outbreaks in a limited population. For instance, in the case of an outbreak associated with a social gathering, guests can be asked about the foods they ate and then about their symptoms. The risk of symptoms associated with each food can be calculated and can indicate the culprit. In cases where the population at risk is larger or more dispersed, it can be difficult to identify all those at risk, so case-control studies may be necessary. Clinicians may be required to contribute in case-finding or by providing data on their patients. (See Glossary for definitions of COHORT STUDY and CASE-CONTROL STUDY).

Dr. Snow and the pump

John Snow was a British physician considered by many to be the father of epidemiology. He identified the source of the London cholera outbreak of 1854 that killed 500 people over a ten day period. Snow plotted the location of all the cholera deaths on a map of central London and interviewed seventy-seven surviving cases. He noticed that a water pump located on Broad Street was in the centre of the area where most cases lived. Cases in outlying areas also tended to draw their water on Broad Street, ironically because of its reputation for pure water.

Dr. John Snow
Figure 11.3a Dr. John Snow

Figure11.3 John Snow's map of cholera cases in central London
Figure 11.3b John Snow’s map of cholera cases in central London

Although, at the time, cholera was thought to be caused by bad air, Snow’s work led him to suspect an association with water. He convinced the local council to remove the handle of the Broad Street pump and, according to legend, this stopped the epidemic. In fact, although Snow’s work was a breakthrough in surveillance and intervention, the epidemic was almost over by the time the handle was removed.

Stage 5: Draw conclusions and re-adjust hypothesis and control measures

Once the cause of the outbreak has been confirmed, the initial control measures may need to be adjusted. If the source cannot be identified, more epidemiological detective work may be required.

Stage 6: Plan for long-term prevention and control

Outbreak investigations should be designed to indicate the possible long-term preventive actions and ways of improving response to future similar outbreaks.

Legislative controls are sometimes needed. For instance, the 2008 outbreak of listeriosis in Canada resulted in discussions about legislative control on food manufacturers. At other times, adequate application of controls and guidelines is needed. For instance, in the case of the water borne illnesses in the Kashechewan First Nations community (see Chapter 10 Illustration box: An environmental hazard in a First Nations Community), recommendations to prevent further problems included upgrading the water treatment plant as well as the water treatment processes, training of personnel, and improved procedures for identifying and correcting problems.


Immunization increases the body’s resistance to illness using immunoglobulins. Antibodies are passed from mothers to their children during pregnancy when they cross the placental barrier, and during lactation when they are secreted in breast milk. They can also be acquired via blood transfusion and by the administration of immune globulin after exposure to disease. These are examples of passive immunization. Antibodies acquired by passive immunization wane rapidly and no memory of the antigen is retained so the protection it offers does not last.

Active immunization uses the body’s own defences to produce a longer lasting protection. It is a response to the introduction of antigens into the body. Natural infection with an immunogenic microbe (natural antigen) can give lasting immunity: vaccines (man-made antigens) do also. Immunity generally declines after active immunization, although at a slower rate than after passive immunization and the immune system retains a ‘memory’ of the antibody so it can mount a rapid response when it meets the antigen again. At the time of writing vaccines are available for 26 pathogens, but this number is likely to increase as there are others whose development WHO is supervising.11 Furthermore, other researchers are responding to emerging infectious disease by trying to develop vaccines against them.

Two vaccines not in the WHO “pipeline”

SARS vaccine
Since SARS emerged in 2002 to 2003 a number of vaccines against it have emerged. Some have reached the level of being tested in humans. However, its safety needs to be assessed and, until another large scale outbreak occurs it will be difficult to fully test its efficacy and effectiveness.12

Ebola vaccine
A number of Ebola vaccines were in development even before the 2014 outbreak in West Africa. Some of these were shown to be safe and highly immunogenic. At the time of writing evidence of vaccine safety, efficacy and effectiveness is still being generated

Vaccination is effective and also one of the safest interventions in medicine. Vaccines are absolutely contraindicated in only three situations. The first is when there has been an anaphylactic reaction to a previous dose of the same vaccine. The second and third apply to live attenuated vaccines only: these should not be given to people who are immunosuppressed and they should not be given to pregnant women. Most reactions to vaccine are minor and do not preclude giving the same vaccine subsequently. Such reactions include pain or swelling at the injection site, rash or fever after injection. As most vaccines are given to children when rashes and fevers are common, it can be difficult to decide if reactions are due to the vaccine or if they would have happened anyway. More serious reactions can be allergic or neurological. While these are not absolute contraindications to future vaccinations, they should cause careful assessment of the patient and the risks and benefits of future vaccination must be analysed.

Some vaccine side-effects

Whooping cough & neurological disorders
Whole cell pertussis vaccine has been available and widely used since the 1940’s. However, it was commonly associated with several adverse effects including local swelling and pain and also with systemic effects such as drowsiness, fretfulness and anorexia. Rarely there were more severe effects such as convulsions, certain neurological manifestations and encephalopathy. Experts still do not agree if the more severe side effects are related to the vaccine. Nonetheless, in view of the concern, an acellular vaccine was developed.13 Although the acellular vaccine is slightly less effective and more expensive, it has largely supplanted the whole cell vaccine in countries that can afford it.

Flu and GBS
In 1976 – 1977 a swine flu vaccine was associated with Guillain-Barré syndrome. There was approximately one case for every 100,000 people vaccinated. This association had not been noted before. In the years since then, very small increases in the incidence of Guillain-Barré syndrome have been associated with flu vaccines, however much larger increases have been associated with influenza infection.14

No vaccine provides 100% protection. Measles vaccine, one of the most effective, has a protection rate of between 85% and 95%; the rate for oral cholera vaccine is just over 65%. However, if the disease can be transmitted only between people and if enough people in a population are vaccinated, vaccines can break the chain of transmission and protect even those in whom the vaccine is not effective. This is known as herd immunity and can be useful in protecting people who have an absolute contraindication to vaccine – vaccinating people who come into contact with them can protect them even if they cannot be vaccinated themselves. Furthermore, if the chain of transmission can be broken, it may be possible to eliminate the disease altogether. Smallpox, a highly virulent infection, was eliminated in 1979.15 In 2015, polio was endemic only in two countries: Afghanistan and Pakistan. Unfortunately, as long as vaccine preventable disease is present somewhere in the world, it can travel quite easily so that global vaccination levels must remain high until the disease is completely eliminated.

Most developed countries are able to maintain high rates of childhood vaccination. Poorer countries face economic, geographic and political barriers to providing vaccines. As illustrated by the outbreaks of diphtheria during and after the break-up of the Soviet Union, war, civil unrest and the associated population movements disrupt vaccination programmes and create opportunity for increased transmission.16 Myths and misconceptions about vaccination disrupt vaccine uptake. Due to public anxiety about whooping cough vaccine in 1975 coverage fell from 80% to 60% in the United Kingdom. This was followed by whooping cough epidemic, which resulted in 12 deaths in 1978.17  Similarly, an article published in the Lancet in 1978 created a scare about a link between measles vaccine and autism spectrum disorder. Although this article was later shown to be fraudulent and subsequent studies failed to show any link, fears about vaccine persist. Other arguments against vaccination are also put forward: see table 11.1 for the more common ones.

Table 11.1: Common myths about vaccine and the real situation adapted from the World Health Organisation18

Common myths The real situation
Vaccine-preventable diseases had already begun to disappear before vaccines were introduced. True, better housing, nutrition, hygiene and sanitation had reduced the impact of disease, but the diseases still exist (along with poor housing, diet, etc.) and vaccine is the most effective way of protecting those at risk against them.
The majority of people who get disease have been vaccinated True: this is because vaccine uptake is high and protection is not 100%. However, those who have been vaccinated usually have a milder form of the disease.
Certain batches of vaccines are associated with higher incidence of adverse events. More reports of adverse events often means better reporting. The vaccine surveillance system is in place so that adverse events can be quickly recognised and linked to a vaccine batch number. Batches with greater than expected adverse events are recalled.
Vaccines cause many harmful side effects. Vaccines are among the safest interventions available. Many cause minor side effects such as soreness where the vaccine was injected. Most cause so few serious side-effects their incidence is difficult to measure.
Vaccine-preventable diseases have been virtually eliminated from the country, so there is no need for vaccination. It only takes one infected traveller from a country where the disease is occurring to cause an outbreak here.
Multiple vaccinations can overload the immune system. If the immune system can manage the daily onslaught of the numerous antigens it comes into contact with, a few extra in the form of vaccines will not harm it.

Infectious Disease Transmission

Our environment is full of microbes – fungi, protozoa, bacteria and viruses – that can infect us and cause an infectious disease. All that is needed is a source of infection, a mode of transmission and a susceptible person with a portal of entry into the body.

The microbes that cause infectious disease are found the soil, air, water and food – as are the causes of environmental disease (see Chapter 10). They can also inhabit other people, animals, insects and they can contaminate inanimate objects, medications and medicinal fluids.

The body has a number of ways of protecting itself against infectious disease. Intact skin prevents microbes entering the body. Mucous in the respiratory system tends to trap microbes while the cilia push them back up the respiratory tract. In the digestive system, gastric acid inactivates many potential pathogens. Finally, if a pathogen has succeeded in getting though these barriers the immune system launches its response. A healthy lifestyle and good living conditions improve the effectiveness of these barriers, while active immunization provides protection against specific diseases.

Figure 11.4 Ways that infections are transmitted.19
Figure 11.4 Ways that infections are transmitted.19

Infections can be spread directly from an infected or colonised person to a susceptible person through direct contact with skin or mucosa or by direct contact with blood or other body fluids: for example, shaking hands, kissing, sexual contact, and sharing needles.

Indirect contact transmission can happen when microbes remain viable in the environment at least long enough to be transmitted. They may have settled on a surface to be picked up by a susceptible person. Variations on contact transmission include transmission by a vector – a living organism capable of transmitting the infection (the hands of healthcare personnel may be considered as vectors or as agents of indirect contact spread). Other vectors would include, for instance, the mosquitoes that spread malaria or Zika virus. Finally, transmission by contaminated medications or other agents used in patient care could also be considered as a form of spread by indirect contact.

Airborne particles also transmit disease. Particles can be generated by coughing or sneezing or by some medical procedures. The size of these particles and the force with which they are generated determine how far they can travel. Usually, particles from about 50 to 100 μm do not travel more than 2 metres from their source, those between 10 and 50 μm can travel within a normal sized sick room and those less than 10 μm can travel outside the room. The resilience of the organism determines how long the particles remain infectious. For instance, Bordetella pertussis becomes nonviable after a short period in the air so that its spread is by droplets. Mycobacterium tuberculosis is resilient enough to be spread in very small airborne particles.

Microbes may use more than one method of transmission – those that are expelled in droplets may land on surfaces and remain there until the susceptible person thus are passed on by indirect contact.

Other organisms, normally present in the environment, cause infectious disease that is generally not transmissible from person to person. Examples include Clostridium tetani, present in the soil, which can enter the body via a penetrating wound. Legionella, also from the soil, can contaminate humidifiers and water systems whence they become airborne, enter the respiratory tract and cause Pontiac fever or Legionnaires’ disease.

The results of coming into contact with a microbe can be:

  • Colonisation – the microbe lives and may multiply but does not cause a reaction
  • Infection – the microbe causes a reaction that may be clinical with associated signs and symptoms or subclinical with only biochemical indication of infection
  • Carrier state – in spite of the host’s reaction and an improvement in the clinical condition, the infection remains and may be passed on.

Terms relating to transmission of infection

Vector – an insect that transmits an infectious disease. The pathogens that cause vector-borne diseases usually require the insect in order to complete their lifecycles. Example would include malaria, the Zika virus or Yellow Fever virus.

Fomite – an inanimate object that becomes contaminated and thus transmits disease. An example would be the poorly cleaned needles of a careless tattoo artist.

Zoonosis – a disease that can be transmitted from animal to man (and vice versa). Examples include campylobacter, anthrax, Ebola, rabies and avian influenza. While prions that cause Bovine spongiform encephalitis are not usually considered to be microbes, the illness can still be classified as a zoonosis.

Anthrax on a Scottish island

For well over a century Bacillus anthracis has been investigated as a possible biological weapon: it can remain viable in unfavourable environments, it is easily dispersed and inhalation of spores can result in serious illness.

In 1942, the British tested Anthrax bombs on the small, uninhabited Scottish island of Gruinard. In the 1980s, the level of contamination was falling, but it was estimated that the island would remain contaminated for another 50 years. Intense political pressure led to the decision to decontaminate it by irrigating with formaldehyde and seawater. Gruinard was declared safe in 1990, more than 50 years after the last test was carried out.

Preventing Transmission of Infectious Disease

Prevention of infectious disease transmission starts with the epidemiological triad – the agent-host-environment described in Chapter 2. Interventions include:


  1. Destruction or weakening of the agent outside the body (e.g., by cleansers, incineration)
  2. Destruction or weakening of the agent inside the body (e.g., use of appropriate antibiotic or antiviral for an appropriate length of time)


  1. Physical barriers (e.g., masks, long pants when hiking in regions that have Lyme Disease)
  2. Decrease opportunity for transmission (e.g., by isolating infective cases, quarantine of contacts, coughing into one’s sleeve, prompt adequate treatment of cases)
  3. Increase specific resistance (e.g., immunization, prophylaxis)
  4. Increase general resistance (e.g., nutrition, exercise)


  1. Hygiene and sanitation (e.g., clean water, sewers, adequate housing)
  2. Vector control (e.g., drainage of mosquito breeding sites, hand hygiene, and respect of infection control measures by health care providers)
  3. Health programmes (e.g., free immunization programmes with call/recall system, partner notification for sexually transmissible infections).

A geographer’s perspective

Jacques May, a medical geographer, was the first to propose the ecological model of health. He viewed disease as a reduction in survival caused by maladjustment of a person to the environment.20 The capacity of individuals to adjust to their environment is genetic, although it is generally mediated by the traits and behaviours associated with their culture. As an example, May cited the high haemoglobin levels found in people who live at high altitude. For them high haemoglobin is a necessary adaption to low oxygen pressure. If a similar haemoglobin level were found in someone living at sea level, it could indicate disease.

These ideas led to the development of May’s model of the elements of transmissible disease, which include:21

  • Agent: causes the disease (e.g., Plasmodium causing malaria);
  • Vector: transmits the agent to the host (e.g., Anopheles mosquito in malaria);
  • Geogens: physical environmental factors (e.g., humidity, temperature, vegetation which allow the survival of Plasmodium and Anopheles);
  • Host: susceptible human.

Figure 11.5 May’s model of transmissible disease, showing the epidemiological triangle situated in the environment.
Figure 11.5 May’s model of transmissible disease, showing the epidemiological triangle situated in the environment.

In developing preventive strategies for specific diseases, it is important consider the following characteristics:

  • The incubation period: the delay between infection and appearance of first symptoms or signs. People who have been in contact with infectious disease should be observed for symptoms during this period. The incubation period can help in differential diagnosis. For instance, a case of gastroenteritis that occurs rapidly after eating a suspect food is likely to be caused by a toxin, such as that produced by Staphylococcus, whereas a delay of 6 hours or more indicates an infectious agent, such as Salmonella.
  • Window period: the time between infection and when the infection can be detected. Some diseases, for instance HIV infection, have a long window period, but can be transmitted during it. In the early stages of HIV infection, negative tests can falsely reassure the patient, who may continue risky behaviour, putting others at risk.
  • Period of communicability: the time during which a person can transmit a pathogen. To prevent transmission, appropriate precautions should be taken through the whole of this period. Some organisms, such as the hepatitis B virus or Salmonella typhi can cause a chronic carrier state in which the patient, having apparently recovered from the disease, continues to transmit it to susceptible people.
  • Attack rate: the proportion of exposed persons who become infected. This is a marker of infectivity, which refers to the ability of an organism to invade, survive, and multiply in the host. For instance, HIV is a fragile organism and is not very infective. Infection with HIV can be prevented by avoiding intimate contact with infected people. At the other end of the scale, measles is highly infective even before the characteristic rash appears, most non-immune people who come into any contact with a case will develop the disease, so that routine immunization is the best means of reducing transmission.
  • Pathogenicity: the proportion of infected persons who develop clinical disease. For instance, young children can develop antibodies to hepatitis A without showing signs of the disease. This happens far less frequently in adults. So hepatitis A is less pathogenic in children than it is in adults. Another example is Corynebacterium diphtheriae, which produces diphtheria only when it is carrying the bacteriophage that produces the toxin that causes diphtheria.
  • Virulence: the proportion of persons with clinical disease who become severely ill or die (severity). The virus that causes the common cold is not virulent; Ebola virus is.
  • Case fatality rate: of all the persons who contract a disease, the proportion that die of it. The more virulent the disease, the higher the case-fatality rate is likely to be and the more urgent it becomes to control its spread. For this reason, physicians who suspect a virulent disease, such as meningococcal illness, should notify the public health authorities as soon as possible by telephone, whereas the notification of less virulent diseases is not so urgent.
  • Herd immunity: the resistance of a group or community to invasion and spread of an infectious agent. If enough people in the community are immune to an agent, the chain of transmission is very likely to be broken before the agent reaches non-immune people. So, the immunity of the “herd” protects non-immune members. This happens only with agents that are transmitted directly from person to person.

Why “quarantine”?

The word “quarantine” is derived from the Italian quaranta giorni meaning 40 days. In 14th Century Venice, in an effort to stop the spread of plague, ships arriving in port and travellers arriving by land had to stay outside the city for 40 days. Unfortunately the quarantine did not include the rats and their fleas that carried the plague and could transmit it from the quarantined travellers to the citizens.

Problems Associated with Health Care

Nosocomial infections

Hospitals and other health care settings are gathering points for large numbers of people whose immune systems are compromised by stress, illness and medications. These people receive intimate care from many busy carers who are also caring for other ill people. Invasive procedures, such the insertion of catheters and cannulas or surgical operations provide additional portals of entry to the body. The equipment used in some of these procedures is too delicate to be sterilised in an autoclave. For microbes, this is a free for all festival where they can reproduce easily and exchange the latest plasmids and nucleic acids that confer antimicrobial resistance. Even those not normally pathogenic can experience the thrill of causing nosocomial illnesses and prolonged hospital stays.


Nosocomial illness: Illness acquired as a result of health care. The term can refer to infections, the complications of medical procedures or, more rarely, the results of MEDICAL ERROR. It is derived from the Greek word nosokomeion meaning a hospital. This is derived from the Greek word for an illness – nosos.

Infection control in a health care setting begins with estimating the risk of transmission of disease. This takes into account the type of patient and illnesses being treated and the type of care being provided. For instance, facilities offering cancer care need to put more resources into infection control than those offering general outpatient care.

Combatting the spread of infectious disease in health care setting comprises three levels of action;19 engineering, administrative and personal protection. Engineering action refers to adaptations of the physical environment such as accessible hand sanitizer dispensers and wash-hand basins, the provision of single rooms with private toilets, ventilation, surfaces that are easily cleaned etc. Administrative action refers to policies and practices around cleaning and building maintenance as well as ensuring adequate resources, like sterile packs, gowns and masks. It also refers to ensuring that the facility has necessary expertise such as an infection control team, adequate education of personnel on the control of infections and the provision of occupational health services such as immunization and rapid response to needle-stick injuries.

Personal protection lies at level of interaction between patient and carer. At this level routine infection control practice includes:

  • Hand hygiene
  • Assessing the risk of disease transmission before any contact with a patient
  • If a patient is likely to transmit disease to others, educating on how to reduce this likelihood and limiting contact between this patient and others. For instance, in family practice seeing patients with a possible respiratory infection at the end of the day and putting them directly into an examination room to limit their time with others in the waiting room
  • Setting up facilities and patient care in a way that reduces the contact between patients. In hospital practice this might mean separate facilities for accommodating inpatients and ambulatory patients
  • Using appropriate aseptic techniques and single use equipment
  • Using masks, gowns and eye protection where necessary
  • Safety techniques for handling and disposing of sharps
  • Properly cleaning clinics and areas used by patients as well as proper handling of used linen and clinical waste
  • Educating of patients and their families and visitors about the risk of disease transmission and how to avoid it
  • Managing visitors to reduce their exposure to transmissible disease as well as their contact with other patients.

Certain situations require additional precautions to reduce the risk of transmission. The precautions are based on the mode of transmission of the organism. Table 11.2 lists some examples. Young children and others who are unable to comply with restrictions and people with incontinence may pose additional risk and should be treated accordingly.

Table 11.2: Examples of additional precautions for the control of disease transmission

Type of spread Examples of organisms and conditions Patient management Clinician
Contact Antibiotic resistant organisms, e.g. MRSA;
Acute vomiting or diarrhoea;
Identify at triage;
Separate from other patients in the waiting room or in a single room.
Hand hygiene;
Gloves for contact;
Gown if soiling is likely;
Clean and disinfect equipment and surfaces after patient leaves.
Droplet Pertussis;
Identify at triage;
Surgical mask;
Put in a single room; Post alert at entrance;
Respiratory etiquette.
Hand hygiene;
Surgical mask and eye protection for any contact;
Clean and disinfect equipment and surfaces after patient leaves.
Airborne Pulmonary tuberculosis;
Chicken pox.
Identify at triage;
Surgical mask;
Put in a single room, close the door and open the window if possible;
Alert notice at room entrance.
Hand hygiene;
N95 respirator if tuberculosis suspected;
Respirator not required for chickenpox or measles if carer is immune;
Only immune staff should provide care.
Cases of acute respiratory infections, for example croup, RSV, influenza, or pneumonia should be treated as droplet and contact spread.

Dr. Rao calls public health

Once back in his office, Dr. Rao called his local public health unit. He was told that a public health nurse had already visited Mrs. Richards’ long-term care home and that an investigation was underway, but there was, as yet, no clue to its cause. Along with the home’s management team, the public health nurse was looking at ways of isolating affected patients and reassigning staff members so that staff caring for the ill patients would not care for the unaffected patients. The public health officer thanked Dr. Rao for calling.

Antimicrobial resistance

Long before antimicrobials existed, microbes had antimicrobial potential. This became recognised as antimicrobials came into use in the 1920s and 1930s. Until about the 1970s, pharmaceutical science was able to keep ahead by developing new antimicrobials that challenged resistance. The development of antimicrobials effective against resistant strains is now slowing so that antimicrobial resistance has become a worldwide problem.

Several factors encourage the growth of antimicrobial resistance:

  • Inappropriate use: antimicrobials for self-limiting infections, the latest antimicrobial instead of an effective older one, broad spectrum antimicrobials instead of appropriate narrow spectrum ones.
  • Inadequate use: too low a dose, too short a course, missed doses, poor choice of route of administration, anything that can expose infecting microbes to an antimicrobial without eradicating them allows the survival of microbes that have some resistance to the antimicrobial. This is a particular problem with infections such as tuberculosis, which require a patient to continue therapy for several months.
  • Use of antimicrobials in veterinary practice as a preventive measure or for promoting growth: intensive modern farming means that animals are raised in conditions that promote the spread of infection. Antimicrobials are often added to animal feed to prevent the loss of livestock (and profits) to infection and to promote growth (and profits). This continuous, low level dose encourages the emergence of antimicrobial resistant strains which can then infect humans directly or pass resistance on to other microbes which infect humans.

To reduce the spread of resistance, physicians have to curb their prescribing habits and educate patients on the proper use of antibiotics and the reasons for this. Physicians should make sure that their diagnosis is correct. A patient who presents with a cough and a runny nose might not have an infection: she might have an allergy. Even if she has an infection, as long as the symptoms are mild, it is likely to be viral and self-limiting. In neither case are antibiotics indicated. In hospitals, where patients are sicker and antimicrobials are used intensively for prolonged periods, attention to infection control practices reduces the spread of resistance.

Survival of the fittest microbe24

As with all life, microbes are subject to evolutionary pressures. Survival of the strain depends on its efficiency in spreading from one host to another. Microbes whose transmission depends on close proximity between the host and a susceptible person do not benefit from being highly virulent, for they require the host to be fit enough to move around and transport them to new hosts. A good example is the common cold, which does not incapacitate enough to limit the host’s mobility; however, it does irritate the upper respiratory tract, causing the coughing and sneezing that efficiently transmit the agent via droplets. Another example is the female pinworm. She deftly deposits her eggs around a child’s anus along with an allergen that causes itching. The child scratches, the eggs get on the fingers, and are then transferred to toys or other people. Microbes that cause sexually transmitted infections are spread by intermittent close contact so benefit from a different strategy. Gonorrhoea, chlamydia, and syphilis can infect silently while their hosts continue an active sex-life, thus ensuring continued spread.

Pathogens that depend on an intermediate vector (such as a flea or mosquito) for transmission can spread even from an immobilised host as long as the vector remains relatively healthy. These pathogens benefit from reproducing rapidly in their host because the greater the number of microbes the host is carrying, the more likely it is that the microbe will be transferred to a passing vector. Plasmodium can be present in large numbers in the blood stream during the periodic, high fevers that characterize malaria and typically confine the patient to bed. The sicker the patient, the less likely he is to worry about that mosquito feeding on his blood and picking up the plasmodium parasite to transmit to the next victim. Similarly, agents able to survive for long periods outside of human hosts can afford to be virulent. Bacillus anthracis and Clostridium tetani are examples of microbes that survive for years in the soil and can kill people rapidly. Again, their ability to reproduce quickly helps the bacteria spread; the patient dies with a high bacterial load and the bacteria return to the soil to patiently await another host.

In general, a disease that spreads via attendants (such as nurses or physicians who carry the pathogen from patient to patient but are not themselves infected) tends towards high virulence. And yet, if the spread is to continue, the microbe can’t incapacitate the attendant, so the transmitted dose has to be small. A small dose of a less virulent pathogen might not establish infection. Following this logic, creating barriers to the transmission of certain community acquired microbes may eventually reduce their virulence while providing immediate control of outbreaks. This is because in order to overcome the barriers, the microbes have to infect their hosts for longer periods of time, keeping the hosts in good enough health to transmit the infection. In theory, therefore, it may be possible to stimulate organisms to evolve into less virulent forms. It is possible that one of the reasons for the huge impact of the 1918–1919 influenza pandemic was that the virus appeared at a time that favoured the transmission of a virus that would be poorly transmissible under normal social conditions.


We must be wary of feeling that infectious diseases no longer pose a problem. We are surrounded by microbes that are fully capable of inventing ways to escape our interventions. Public health counteracts outbreaks in the population using surveillance to detect them and then implementing the steps of outbreak control. Vaccination is one of the best ways of controlling and, in some cases, even eliminating vaccine-preventable diseases. Health care is the source of nosocomial disease and antibiotic resistance. Clinicians must ensure that they are not increasing the transmission of infectious disease.

Self-test Questions

1. List the steps of outbreak control.

a.     establish the existence of an outbreak;

b.     define what constitutes a case and identify cases;

c.      formulate hypotheses on the causes and implement initial control measures;

d.     test hypotheses through analysis of surveillance data or special studies;

e.      draw conclusions, re-adjust hypotheses and control measures if needed;

f.       plan for long-term prevention and surveillance.

2. What are the advantages of immunization in the control of infectious disease?

Vaccination provides long-lasting protection that reduces a person’s chances of getting the specific disease. It is a form of passive prevention as it does not rely on continued cooperation for its success. It can break the chain of transmission, thus preventing or arresting outbreaks and, if there is no animal reservoir it can even eradicate the disease.

3. A hospital patient has contracted Clostridium difficile. What precautions should be taken?

Clostridium difficile is spread by direct and indirect contact. Indirect spread can occur via carers and via surfaces. Adequate hand-washing or hand-cleansing is essential as is thorough cleaning of the room after an infected patient has been in it.


Reflection Questions

  1. Why have antibiotic-resistant strains of bacteria become a problem in recent years?
  2. What precautions should a clinician take against the spread of infectious disease in a clinic setting?
  3. Why is the common cold not a notifiable disease?
  4. What diseases are notifiable in your province Territory?


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