Public Health Questions and Answers (Epidemiology)

Define Epidemiology.

A. Epidemiology is defined as the distribution and determinants of disease frequency or health events in man. Modern day epidemiology is different from the earlier period when it was just referred to as the study of epidemics. It now includes comprehensive methods for the control of diseases, including non-communicable diseases. Distribution refers to the time; place and personal characteristics of disease while the determinants (what determines disease) are generally characterized as an agent, host, and environmental factors. Since freedom from disease allows an individual to remain healthy, it is also important to find out how and why individuals do not suffer from disease and remain healthy. Such analyses will help in finding solutions to diseases and maintaining good health.


What are the uses of epidemiology?

A. The major uses of epidemiology are:

To assess the magnitude or burden of disease in a community. It, therefore, helps in studying the occurrence of disease in a population.

To assess the health status of communities. It, therefore, helps in establishing a community diagnosis.

To search for determinants of disease. To find out how and why the disease is caused is a major use of epidemiology.

To estimate an individual’s risks and chances of suffering from a disease and to establish the prognosis of an individual suffering from the disease.

To plan comprehensive health services, including specific strategies and ways and means of implementation.

To evaluate strategies and interventions for disease control. Such evaluation helps in identifying weaknesses and suggesting remedial measures for the future. Evaluation of costs and benefits or effectiveness of specific interventions is also an integral use.

To complete the natural history of the disease. In a hospital setting only the terminal cases are seen and how the disease starts and presents in its initial stages is only possible by studying the disease in the community.

To forecast future disease trends.

To identify syndromes.


What is the epidemiological triad?

A. Disease is caused by an interaction between agent, host, and environmental characteristics. When all three are in harmony, health is ensured but maladjustment in their relationships leads to disease. These three factors together constitute the epidemiological triad.


What is the difference between retrospective and prospective studies?

A. Retrospective studies start after a disease has occurred and the investigator looks back in time to find out what agents or characteristics (including habits) the individuals were exposed to and which could lead to disease. Case control studies are examples of retrospective studies. In prospective studies, individuals are identified before the occurrence of the disease and they are followed up to see which individuals develop the disease. The characteristics or exposures of these individuals to disease causing agents are compared with the characteristics of individuals who do not suffer from the disease. This helps in searching for determinants of disease.


What is a case control study?

A. A case control study is an epidemiological study where a group of individuals with a disease is compared with a group of individuals who are not suffering from the disease in terms of specific disease causing exposures. Since the starting point is a group of people who already have suffered from the disease, this is labeled as a retrospective study.

Advantages of case control studies

Relatively quick and easy to undertake.

Relatively cheap to undertake.

The only method is useful in rare diseases. Not enmeshed in problems of follow-up as the data is collected at one point in time. Can be used to study the effect of many exposure variables on a single disease outcome.

Drawbacks of case control studies

Prone to selection and recall bias.

Can’t measure relative risk or provide incidence estimates.

Sometimes the occurrence of the exposure in terms of time, i.e. whether it occurred before the disease may be difficult to decipher.

Can’t be used for rare exposures.

Designing the study is not an easy task.


What is a cohort study?

A.Cohort studies are forward looking, i.e. they look for the development of disease in a group of individuals (the cohort) free of the same at the beginning. The group is followed up over a period of time. During this period some persons will develop the disease under study while others will remain free of the disease. The characteristics (and exposure to disease causing factors) are compared between those who suffer from the disease and those free from the disease. The literal meaning of the term ‘cohort’ refers to a group that shares similar characteristics.

Thus, it implies that one needs to identify groups of populations who are free of the disease being studied and who are similar in all respects, except for the specific exposure variable or characteristic whose effect is related to the disease being studied. These groups are then followed up for the period of time that it takes for the disease to develop.


What is a Randomized Controlled Trial?

A. A randomized controlled trial is an experimental method where individuals are randomly allocated to an experimental or a control group and the effect or response of a drug or intervention is compared between the two groups. The two key features are ‘randomization’ and ‘controlled’. To ensure that there is no bias on part of the investigator, these trials are ‘blinded’ – single blind means the patients do not know what the intervention is; ‘double’ blind means that neither the patient nor the researcher knows what the drug/ intervention package is while ‘triple’ blind means that the analysis team also does not know which is the experimental and which is the control group till the study has been completed.


What is a Community Trial?

A. A community trial is a modification of the clinical trial where instead of individuals being allocated to experimental or control groups, whole communities are randomly allocated to receive specific interventions, and analysis is done for whole communities rather than for individuals in the communities. Vitamin A supplementation trials, iron supplementation, or iodine supplementation trials are all examples of community trials.


 What is a vaccine trial?

A. When a randomized control trial is done to test the efficacy of a vaccine in preventing disease, it is called a vaccine trial.


What is the difference between Descriptive and Analytical Epidemiology?

A. The descriptive methods are mostly concerned with the distribution of disease/health condition, while the analytical methods are concerned with the determinants of disease/health condition.

There are a number of different descriptive methods:

Ecological /correlational studies.

Case reports/case series. – Cross-sectional designs

Analytical studies are concerned with the determinants of disease rather than with the distribution. They focus attention on ways to prove a hypothesis suggested by the earlier types of studies. Based on the analytical method used, they can be categorized into two types:

  • Observational methods


  • Experimental methods.

In the observational methods, the disease phenomenon is not in the grip of the investigator. The clinician – investigator only records the sequence of events as they unfold in front of his eyes. The outcome events are then related to the different input variables to understand the dynamics of disease causation. In experimental studies or intervention studies, the researcher selects a group of patients and then decides how to intervene/treat the group, and then records the differences.

Thus, one group of individuals is labeled as an experimental group and is given some ‘treatment’, while the other group is labeled as the control group among whom either a placebo may be used or nothing may be done.

These studies can be further categorized as follows:

Observational Studies

Experimental Studies

– Case control studies

– Intervention studies

– Cohort studies with/ without control

–Clinical trials (RCT)

– Community trials.


What is sampling?

A. Sampling can be defined as the process of selecting a statistically determined number of subjects from the universe or reference population, which provides an accurate estimate of the problem being studied. Thus, only a proportion of the reference population is covered, but the results approximate the actual prevalence of the disease condition in the reference population. Strictly speaking, the results obtained are applicable only to the population from which the sample was drawn, but the results are generally extrapolated to other populations that are similar to the reference population. A sample is the minimum number of people or units who need to be contacted or examined to obtain statistically acceptable results and thereby permit valid inferences to be drawn.


What are the different sampling methods that you know of?

A. The different sampling methods include simple random sampling, systematic random sampling, stratified random sampling, cluster sampling, multistage sampling, and purposive sampling. Purposive sampling is not used in epidemiological or health sciences as it gives biased results. All other sampling methods are called probability-sampling methods.


What is the difference between simple and stratified random sampling?

A. In this method, every individual has an equal chance of being selected in the sample. The first step involves the enumeration of the total population wherein every individual is sequentially numbered from 1…n (n =last member in the population). The enumeration list of the total population from which the sample is drawn is called the sampling frame. The next stage is the actual selection of the sample, which can be done by different methods like drawing numbers from a hat or picking numbers from a random table, etc. Stratified sampling is done to give the different population subgroups an equal chance of being selected. In this way, male/ female or urban/rural can be given adequate representation to reduce bias.


What is systematic sampling?

A. Systematic sampling is a method of probability sampling, which is used to simplify the selection procedure. After enumerating the units i.e. villages, houses, etc. they are selected at a predetermined interval, i.e. every nth unit. The procedure has the advantage of easy administration in the field but is inferior compared to simple random sampling.


What is cluster sampling? Give examples of where cluster sampling has been used in reproductive and child health.

A. For conducting surveys or epidemiologic investigations on large population groups, the simple random technique is not of much use. This is because the construction of the sampling frame becomes very difficult when there are a large number of individuals to be enumerated. In such situations, groups, rather than individuals, can be selected in the initial stage of the sampling process. In the selection of the final stage units, instead of randomly selecting the individuals to be included, all the individuals in the identified cluster are examined.

EPI vaccine coverage surveys adopted the cluster sampling technique and now this technique is widely used in many situations where coverage/lameness/disability are surveyed.


What is an epidemic?

A. The occurrence of disease clearly in excess of normal expectations is called an epidemic. The number of cases, which should be diagnosed before declaring an epidemic status, depends on the number of cases routinely seen in that area. In an area where a disease has not been seen for many years, even the occurrence of a single case may be sufficient to call it an epidemic. Some people look at the mean (or average) number of cases and if the cases are beyond two standard deviations they call it an epidemic.


Give some examples of epidemic diseases, which have occurred in India recently.

A. The recent outbreak of plague in India where cases were not seen for many years is an example of an epidemic.

HIV/AIDS is also a disease with epidemic proportions. Epidemics of dengue and JE is also reported from time to time.

Epidemic diseases need not necessarily be communicable diseases. Therefore, the WHO also looks at smoking as an epidemic.


What is a pandemic?

A. An epidemic that breaks out across many continents is called a pandemic – i.e. occurring across the world.


Name some pandemic diseases.

A. COVID-19, HIV/AIDS, and smoking can be called modern day pandemics as they have affected millions of people across the world. Cholera was one of the most common diseases, which assumed pandemic proportions. Drug resistant tuberculosis is also a pandemic. The plague was also a pandemic in historical times.


What are endemic diseases? Name some endemic diseases

A. The constant, continuous, or usual presence of a disease in a defined geographic area or delimited territory is called an endemic disease. Hyperendemic refers to a persistent intense transmission in an area while holoendemic means a disease starting early in life and affecting most of the population. An endemic disease may become an epidemic if the number of cases usually seen suddenly increases in proportion. Malaria, tuberculosis, leprosy, filariasis, etc. are examples of endemic diseases.


What is the prevalence rate?

A.The proportion of persons suffering from a specific disease out of the population normally residing in that area, at a particular point in time, is called the prevalence rate. It includes both the new cases as well as the old cases occurring in the area at the point in time when the examination was undertaken. The point in time can be one day or one year or more depending upon how much time it takes to examine the population residing in an area. The persons suffering from the numerator while the population from which they hail is called the denominator. It should be seen that the numerator is part of the denominator. For example, if the female genital discharge is being studied, the denominator should only include the population who are at risk of suffering from a disease i.e. women. Prevalence rate can be represented as a percentage if a disease is common or as per 1000 or 100,000 population if the disease is rare.


 What is the incidence rate?

A. Incidence rate refers to the number of new cases occurring in a population over a specified period of time. The numerator should be part of the denominator as in prevalence rate but unlike as in prevalence rate only new cases are considered. If a case started before the reference period but is continuing to the present, it is not considered in the numerator. E.g. if the reference period is from 1st Jan – 31st Jan 2002, cases, which started before 1st Jan 2002 but are still suffering will not be included in the numerator. The incidence rate is generally depicted as per 1000 or 100,000.


What is the relationship between prevalence and incidence?

A.Prevalence of a disease is the product of incidence and the duration of disease ( P= I xD). Therefore, the prevalence of a disease depends not only on the actual number of people who develop a disease but also on the duration of the disease. For short duration diseases like the common cold, the prevalence and incidence are almost identical while for chronic or longstanding diseases like tuberculosis, leprosy, or blindness the prevalence is always much higher than the incidence.


How does the prevalence of a disease increase?

A.The prevalence of a disease can increase in the following conditions:

The duration of the disease is very long (i.e. chronic conditions)

The level of incidence i.e. the higher the incidence, the larger the prevalence.

Improved methods of diagnosis lead to the detection of a larger number of cases than before.

Availability of effective treatment, which prolongs life such that the individual lives longer while still suffering from the

A sudden migration of cases into an area where the disease was not very common earlier.


How does the prevalence of a disease decrease?

A. A very short duration of the disease (applicable to acute disease conditions).

      1. A very low incidence of disease
      2. Lack of proper diagnostic equipment or skills for the detection of disease.
      3. Diseases with high mortality such that very few individuals survive
      4. Outmigration of diseased individuals


In which disease conditions is prevalence more appropriate and why?

A. Prevalence is most appropriate in long-standing or chronic diseases as the window period in which a diagnosis can be

established is much higher and therefore, cases will not be missed during a survey. On the other hand, short duration

diseases would occur and recover so fast that they cannot be examined (or are missed) at a specific point in time.


In which diseases are incidence more appropriate and why?

A. Incidence is most appropriate in diseases of short duration as such diseases may occur more than once during a reference period and all the episodes may not be captured if the prevalence of the disease were to be measured. E.g. If a study were done on diarrhea, if the number of people suffering from diarrhea in a one-year period was measured, it would always be much lower than the number of episodes of diarrhea that the population staying in that area would suffer.


What do you mean by a primary case?

A.The first case of a disease that occurs in a community/area is called the primary case. Many a time, the primary case may not be recognized as the disease comes to notice much later. Only by historical review can, the possible primary case, be located in such a case. In some diseases like acute conjunctivitis, a number of primary cases may occur almost at the same point in time. In such a scenario, the primary cases are referred to as “Co–primaries”


What do you mean by an index case?

A. The first case, which comes to the attention of the health authorities in an area, is referred to as the index case. Such a case may or may not be the primary case.


What is the Secondary Attack Rate?

A. The secondary attack rate refers to the number of cases occurring among contacts of a primary case within the known incubation period of the disease. The denominator refers to the number of susceptible contacts who are in close touch with the primary case. However, if whether a person among the contacts has previously suffered from the specific disease and developed immunity is not known, then all the contact should be considered in the denominator. Such persons are usually other members of the family/neighborhood/institution who stay with the primary case.


What is a secular trend?

A. If the pattern or trend of disease frequency changes only over many years then it is called a secular trend.


What is a cyclic trend?

A. If the occurrence of disease changes over a short duration of time like a year, it is called a cyclic trend. Most epidemic diseases in India show a cyclic trend. Some diseases change in frequency over seasons and such changes are referred to as seasonal changes –Measles and chickenpox are examples of such diseases.


What is herd immunity? Give examples where it is important.

A. The immune status of a group of people/community is called herd immunity as it is the immune status of the ‘herd’ of people. For many communicable diseases, an outbreak of disease is only possible if the level of immunity is sufficiently low and there are a large number of susceptibles in the population. In diseases like poliomyelitis, diphtheria, measles, etc., herd immunity plays an important role. However, in a disease like tetanus or rabies where every individual is at risk unless specifically protected, herd immunity plays no role.


What is a nosocomial infection?

A. An infection occurring in a patient in a hospital or other healthcare facility and in whom it was not present or incubating at the time of admission or arrival at a healthcare facility is called a nosocomial infection. It refers to diseases transmitted from a hospital. Usually, such infections are more difficult to manage as they are generally resistant to most of the common antibiotics. Nosocomial infections also include those infections, which were contracted in the hospital but manifested after discharge, and also infections suffered by staff members if they contracted the infection from the hospitalized patients.


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