Infectious disease epidemiology

Introduction

Infectious disease epidemiology is a fundamental part of the whole of epidemiology. In fact, the subject of epidemiology originally developed from the study of epidemics of infectious diseases. There is a renaissance in the study of communicable diseases, stimulated by;

1. changes in the pattern of communicable diseases,
2. the discovery of “new” infections, and
3. the possibility that some chronic diseases have an infective origin.

The development of vaccines and antibiotics was not followed, as predicted, by the virtual disappearance of infectious disease. Its prevention and control needs epidemiological knowledge and experience. This section focuses on infectious disease epidemiology.

Terms used in epidemiology

Infection

The entry and development or multiplication of an infectious agent in the body of man or animals. It also implies that the body responds in some way to defend itself against the invader, either in the form of an immune response or disease. An infection does not always cause illness.

Contamination

The presence of an infectious agent on a body surface; also on or in clothes, bedding, toys, surgical instruments, dressings, or other inanimate articles or substances including water, milk, and food. Contamination on a body surface does not necessarily imply a carrier state.

Infestation

For persons or animals, the lodgement, development, and reproduction of arthropods on the surface of the body or in clothing, e.g., lice, itch mites. Some authorities use the term to describe the invasion of the gut by parasitic worms, e.g., ascariasis. Infested articles or premises are those which harbor or give shelter to animal forms, especially arthropods and rodents.

Host

A person or other animal, including birds and arthropods, that provide subsistence or lodgement to an infectious agent under natural conditions. An obligate host means the only host, e.g., man in measles and typhoid fever. Hosts in which the parasite attains maturity or passes its sexual stage are primary or definitive hosts; those in which the parasite is in a larval or asexual state are secondary or intermediate hosts. A transport host is a carrier in which the organism remains alive but does not undergo development.

Infectious disease

A clinically manifest disease of man or animals resulting from an infection.

Contagious disease

A disease that is transmitted through contact is called a “contact-transmitted disease” or “contact-transmissible disease. Examples include scabies, trachoma, STD and leprosy.

Communicable disease

An illness due to a specific infectious agent or its toxic products capable of being directly or indirectly transmitted from man to man, animal to animal, or from the environment (through air, dust, soil, water, food, etc.) to man or animal.

Non communicable disease

Non-communicable diseases (NCDs), also known as chronic diseases, are medical conditions or health problems that are not caused by infectious agents and are typically of long duration. These diseases tend to progress slowly over time and are often influenced by a combination of genetic, environmental, and behavioral factors. Common examples include cardiovascular diseases (such as heart disease and stroke), cancer, chronic respiratory diseases (chronic obstructive pulmonary disease and asthma), diabetes, and mental health disorders.

Epidemiology

The branch of medicine that deals with the incidence, distribution and possible control of diseases and other factors relating to health.

Epidemic

(Epi upon; demos = people). The “unusual” occurrence in a community or region of disease, specific health-related behavior (e.g., smoking) or other health related events (e.g., traffic accidents) clearly in excess of “expected occurrence”. The amount of disease occurring in the past, in the absence of an epidemic, defines the “expected” frequency. Some use the term “outbreak” for a small, usually localized epidemic in the interest of minimizing public alarm, unless the number of cases is indeed very large.

Endemic

 Endemic refers to the constant presence of a disease or infectious agent within a given geographic area or population group, without importation from outside; it may also refer to the “usual” or expected frequency of the disease within such an area or population group. For instance, the common cold is considered endemic because it is consistently present within the population.

Sporadic

The term “sporadic” describes diseases or occurrences that are scattered about, occurring irregularly and infrequently over time and space, without a recognizable common source of infection. Examples include polio, tetanus, herpes zoster, and meningococcal meningitis. A sporadic disease may serve as the initial point of an epidemic when conditions favor its spread. Many zoonotic diseases are characterized by sporadic transmission to humans.

Pandemic

An epidemic typically affects a large proportion of the population, spreading over a wide geographic area such as a section of a nation, the entire nation, a continent, or even the world. Examples include influenza, Acute Respiratory Syndrome (SARS), COVID-19 etc.

Exotic

Diseases which are imported into a country in which they do not otherwise occur, as for example, rabies in UK.

Zoonoses

An infection or infectious disease transmissible under natural conditions from vertebrate animals to man. May be enzootic or epizootic e.g., rabies, plague, bovine tuberculosis, anthrax, brucellosis, salmonellosis, endemic typhus, hydatidosis, etc. In recent years several new zoonoses have emerged, e.g. Monkey pox, Lassa fever, etc.  

Epizootic

An outbreak (epidemic) of disease in an animal population (often with the implication that it may also affect human populations). Only a few zoonotic agents cause major epidemics. Notable among these are the agents of anthrax, brucellosis, rabies, influenza, Rift valley fever, Q fever, Japanese encephalitis and equine encephalitis. The study of epizootic diseases is given the name of epizootiology.

Epornithic

An outbreak (epidemic) of disease in a bird population.

Enzootic

An endemic occurring in animals e.g., anthrax, rabies, brucellosis, bovine tuberculosis, endemic typhus and tick typhus.

Nosocomial infection

Nosocomial (hospital acquired) infection is an infection originating in a patient while in a hospital or other health care facility. It denotes a new disorder (unrelated to the patient’s primary condition) associated with being in a hospital. That is, it was not present or incubating at the time of admission or the residual of an infection acquired during a previous admission. It includes infections acquired in the hospital but appearing after discharge, and also such infections among the staff of the facility. Examples include infection of surgical wounds, hepatitis B and urinary tract infections.

Opportunistic infection

This is infection by an organism(s) that takes the opportunity provided by a defect in host defense to infect the host and hence cause disease. The organisms include Herpes simplex, Cytomegalovirus, Toxoplasma, M. tuberculosis, pneumocystis, etc. (For example, opportunistic infections are very common in AIDS). Infection by an organism that is not normally pathogenic, but can cause disease if resistance is lowered.

Iatrogenic (physician-induced) disease

Any untoward or adverse consequence of a preventive, diagnostic or therapeutic regimen or procedure, that causes impairment, handicap, disability or death resulting from a physician’s professional activity or from the professional activity of other health professionals. The disease may be serious enough to prolong the hospital stay, require special treatment or actually threaten life. Most of the episodes are related to drug therapy, immunization or diagnostic procedures, e.g. reactions to penicillin and immunizing agents, aplastic anaemia following the use of chloramphenicol, childhood leukaemia due to prenatal X-rays, hepatitis B following blood transfusion, etc. These are all preventable.  

Surveillance

The continuous scrutiny of the factors that determine the occurrence and distribution of disease and other conditions of ill health. Surveillance is essential for effective control and prevention, and includes the collection, analysis, interpretation and distribution of relevant data for action.

Eradication

Termination of all transmission of infection by extermination of the infectious agent through surveillance and containment. Eradication is an absolute process, an “all or none” phenomenon, restricted to termination of an infection from the whole world. It implies that disease will no longer occur in a population. To-date, only one disease has been eradicated, that is smallpox.

The term elimination is sometimes used to describe “eradication” of disease (e.g., measles) from a large geographic region or political jurisdiction. In the state of our present knowledge, diseases which are amenable to eradication are measles, diphtheria, polio and guinea worm.

Elimination

It is the process of getting rid of something whether it’s waste, errors or the competition. Elimination come from the Latin word limen which means threshold.

Virulence

Virulence refers to the degree of pathogenicity, or the ability of a microorganism (such as a virus, bacterium, fungus, or parasite) to cause disease in a host. Microorganisms with high virulence are more likely to cause severe illness or death in the host organism, while those with lower virulence may cause milder symptoms or no symptoms at all.

Epidemiology

Epidemiology is the basic science of preventive and social medicine. Although of ancient lineage, it made only slow progress up to the start of 20th century. Epidemiology has evolved rapidly during the past few decades. Its ramifications cover not only study of disease distribution and causation, but also health and health-related events occurring · in human population. Modern epidemiology has entered the most exciting phase of its evolution. By identifying risk factors of chronic disease, evaluating treatment modalities and health services, it has provided new opportunities for prevention, treatment, planning and improving the effectiveness and efficiency of health services. The current interest of medical sciences in epidemiology has given rise to newer off-shoots such as infectious disease epidemiology, chronic disease epidemiology, clinical epidemiology, serological epidemiology, cancer epidemiology, malaria epidemiology, neuro epidemiology, genetic epidemiology, occupational epidemiology, psychosocial epidemiology, and so on.

History

Epidemiology began with Adam and Eve, both trying to investigate the qualities of the “forbidden fruit” Epidemiology has grown rapidly during the past few decades. It has now become firmly established in medical education.

Epidemiology is derived from the word epidemic (epi=among; demos=people; logos=study) which is a very old word dating back to the 3rd century B.C. The foundation of epidemiology was laid in the 19th century, when a few classic studies made a major contribution to the saving of life.

Epidemiology has evolved significantly since its inception, with key milestones marking its progress. In the mid-19th century, figures like John Snow pioneered the field with his investigation into the cholera outbreak in London, showcasing the power of epidemiological methods in disease control.

The 20th century saw further advancements as epidemiologists tackled major public health challenges such as the eradication of smallpox and the control of infectious diseases like polio through vaccination campaigns. Epidemiological studies also played a crucial role in identifying risk factors for chronic diseases such as heart disease, cancer, and diabetes, leading to preventive measures and improved health outcomes.

Definition

“The study of the distribution and determinants of health-related states or events in specified populations, and the application of this study to the control of health problem”. (John M. Last, 1988)

Aims of epidemiology

According to the International Epidemiological Association (IEA), epidemiology has three main aims:

1. to describe the distribution and magnitude of health and disease problems in human populations
2. to identify aetiological factors (risk factors} in the pathogenesis of disease; and
3.  to provide the data essential to the planning, implementation and evaluation of services for the prevention, control and treatment of disease and to the setting up of priorities among those services.

The ultimate aim of epidemiology is to lead to effective action:

1. to eliminate or reduce the health problem or its consequences; and
2. to promote the health and well-being of society as a whole.

Uses of epidemiology

Morris has identified seven distinct uses of epidemiology, five of which extend epidemiology beyond the search for causes of disease and bring it closer to day-to-day concerns of modern medicine. These are:

1. To study historically the rise and fall of disease in the population

It is well known that the health and disease pattern in a community is never constant. There are fluctuations both over short and long periods of time. For example, the first contribution of epidemiology to the study of coronary heart disease was that it was an “epidemic”. Later many others such as accidents, cancer and diabetes were found to be “epidemic”. As old diseases (e.g., smallpox) are conquered, new ones (e.g. Lassa fever, AIDS) have been identified, in which epidemiology has played a major role.

2. Community diagnosis

 Community diagnosis generally refers to the identification and quantification of health problems in a community in terms of mortality and morbidity rates and ratios, and identification of their correlates for the purpose of defining those individuals or groups at risk or those in need of health care. By quantification of health problems, we lay down priorities in disease control and prevention. Secondly, quantification of morbidity and mortality can serve as a benchmark for the evaluation of health services at a later date. Thirdly, the quantification of health problems can be a source of new knowledge about disease distribution, causation and prevention.

3. Planning and evaluation

Planning is essential for a rational allocation of the limited resources. For example, in developing countries, too many hospitals have been built and equipped without knowledge of the particular disease problems in the community. Epidemiologic information about the distribution of health problems over time and place provides the fundamental basis for planning and developing the needed health services and for assessing the impact of these services on the people’s problems..

Evaluation is an equally important concern of epidemiology. Any measures taken to control or prevent a disease must be followed by an evaluation to find out whether the measures undertaken are effective in reducing the frequency of the disease. Evaluation of a control method such as hepatitis vaccine requires more than the demonstration of its effectiveness in reducing disease frequency.

4. Evaluation of individual’s risks and chances

One of the important tasks of epidemiologists is to make a statement about the degree of risk in a population. Besides the incidence rate and specific rates which are measures of absolute risk, the epidemiologists calculate relative risk and attributable risk for a factor related to or believed to be a cause of the disease. The risk of bearing a mongol child and of some hereditary disorders are classic examples of evaluating individual’s risks and chances. The risk assessment for smokers and non-smokers, for selected causes of death (e.g., cancer CHD) is another well-known example.

5. Syndrome identification

Medical syndromes are identified by observing frequently associated findings in individual patients. It is worth recalling that, although approximately 3000 so-called syndromes are described in the contemporary paediatric literature, a primary defect is known only in about 20 per cent of these. Epidemiological investigations can be used to define and refine syndromes. By observation of groups, such studies have been able to correct misconceptions concerning many disease syndromes. For example, there was less appreciation of the two main types of peptic ulcer (gastric and duodenal) till 1920. But the “poverty” gradient in the certification of the gastric ulcer and its absence in duodenal ulcer led to differentiation of gastric and duodenal ulcers.

6.  Completing the natural history of disease

Epidemiology is concerned with the entire spectrum of disease in a population. The picture of disease constructed on the basis of hospital patients is quite different from that found in the community. The epidemiologist by studying disease patterns in the community in relation to agent, host and environmental factors is in a better position to fill up the gaps in the natural history of disease than the clinician. For example, an outstanding contribution by epidemiology to the natural history of atherosclerosis is the recognition that one­ third to two-thirds of all deaths due to ischemic heart disease are sudden, i.e., occur in less than one hour. Hospital studies could never have come to this conclusion, for most victims do not reach the hospital.

7. Searching for causes and risk factors

Epidemiology, by relating disease to inter-population differences and other attributes of the population or cohorts examined, tries to identify the causes of disease. The contributions of epidemiology have been many in this regard. Numerous examples can be cited: epidemiological studies have incriminated that rubella is the cause of congenital defects in the newborn, that thalidomide is a teratogenic agent, cigarette smoking is a cause of lung cancer, exposure of premature babies to oxygen is the cause of retrolental fibroplasia, etc. in the case of chronic disease, hopes of finding a single cause remains unfulfilled, but an important conceptual change has occurred – that is, search for risk factors.

Epidemiological approach

The epidemiological approach to problems of health and disease is based on two major foundations:

1. Asking questions
2. Making comparisons.

1. Asking questions

Epidemiology has been defined as “a means of learning or asking questions and getting answers that lead to further questions”. For example, the following questions could be asked:

Related to health events

1. What is the event? (the problem)
2. What is its magnitude?
3. Where did it happen?
4. When did it happen?
5.  Who are affected?
6. Why did it happen?

Related to health action

1. What can be done to reduce this problem and its consequences?
2. How can it be prevented in the future?
3. What action should be taken by the community?  By the health services? By other sectors? Where and for whom these activities be carried out?
4.  What resources are required? How are the activities to be organized?
5. What difficulties may arise, and how might they be overcome?

Answer to the above questions may provide clues to disease etiology, and help the epidemiologist to guide planning and evaluation.

2. Making comparisons

The basic approach in epidemiology is to make comparisons and draw inferences. This may be comparison of two (or more groups) one group having the disease (or exposed to risk factor) and the other group(s) not having the disease (or not exposed to risk factor}, or comparison between individuals. By making comparisons, the epidemiologist tries to find out the crucial differences in the host and environmental factors between those affected and not affected. In short the epidemiologist weighs, balances and contrasts. Clues to aetiology come from such comparisons.

One of the first considerations before making comparisons is to ensure what is known as “comparability” between the study and control groups. In other words, both the groups should be similar so that “like can be compared with like”. For facts to be comparable, they must be accurate, and they must be gathered in a uniform way. For example, the study and control groups should be similar with regard to their age and sex composition, and similar other pertinent variables. The best method of ensuring comparability, in such cases, is by randomization or random allocation. Where random allocation is not possible (as in case control and cohort studies} what is known as “matching” is done for selected characteristics that might confound the interpretation of results.

Basic measurements in epidemiology

Epidemiology focuses, among other things, on measurement of mortality and morbidity in human populations. The scope of measurements in epidemiology is very broad and unlimited and includes the following:  

1. Measurement of mortality
2. Measurement of morbidity
3. Measurement of disability
4. Measurement of natality
5. Measurement of the presence, absence or distribution of the characteristic or attributes of the disease
6. Measurement of medical needs, health care facilities, utilization of health services and other health-related events
7. Measurement of the presence, absence or distribution of the environmental and other factors suspected of causing the disease, and
8. Measurement of demographic variables.

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Sources: Park, K. (2021). Park’s Textbook of Preventive and Social Medicine (26th ed.). Bhanot Publishers.