Many students wonder why they have to learn epidemiology during their medical training. In this article, I will attempt to explain why without getting into technicalities.

Historically, epidemiology was the study of epidemics- of which there were plenty till the middle of the 20th century. Most of our efforts were focused on the prevention and control of communicable diseases responsible for epidemics.

However, with advances in living conditions, environmental sanitation and hygiene, immunization, and pharmacological therapeutics (particularly antimicrobials), communicable diseases declined as major public health problems. Gradually, non-communicable diseases became important causes of morbidity and mortality.

Through all of this, epidemiology helped make sense of health and disease. Epidemiology is concerned with the health of populations, and epidemiological investigations helped determine how to prevent and control diseases like cholera long before the cholera bacillus was discovered- a remarkable achievement.

Careful and meticulous epidemiological investigations are responsible for the discovery of several new diseases and the means to control them. Whenever we are faced with a new disease, epidemiological investigations help provide crucial initial insights into disease transmission, for instance. Although the initial phase of investigation into a new disease is messy and chaotic, once the data are analyzed, there is greater clarity on how to deal with the situation.

Why investigate diseases or health conditions in the first place? Initially, sovereign states/ nations were interested in ensuring the health of their citizens because healthier people meant more people to fight in the army. However, health was largely a personal matter. Later, states began to take responsibility for the health of their citizens. This was driven by both military and financial concerns. Since states were involved in health, they began wondering how to enhance health and decrease mortality. Enter epidemiology.

Epidemiological investigations generated empirical evidence that in turn guided public policy. Major sanitary reforms were the result of landmark epidemiological investigations showing the role of water in transmitting diseases. Gradually, focus shifted to other (Non-Communicable) diseases, and scientists began investigating the relationship between suspected risk factors and diseases.

[Sometimes the evidence was misinterpreted to suit popular beliefs- like when increased incidence of cholera was blamed not on contaminated water, but the foul gases emanating from decomposing organic matter in water because the miasma theory was popular at the time! However, eventually, we got the interpretation right. This point highlights the trial and error inherent in science- we don’t know we are right or wrong until there is a large amount of evidence in support of one or other position. The good thing is that we expect to be proved wrong in science if we truly are wrong. In fact, the whole peer-review process is based on one’s peers punching holes in one’s research. Scientists are very hard to convince because they are sceptics.]

Investigating the relationship between risk factors and disease(s) is not easy. First, one must identify the risk factors. Sometimes, risk factors are easily identifiable- like tobacco chewing and oral cancer (we observe that cancer often appears where the tobacco quid is placed). On other occasions, it is not so straightforward- like finding that using hair dye increases the risk of breast cancer. Once risk factors have been identified, epidemiological studies must identify those factors that have a causal relationship with the health condition/ disease by carefully eliminating alternative explanations (this is similar to separating the signal from noise).

Even when a causal relationship cannot be demonstrated, suspected risk factors must continue to be investigated to determine the magnitude of their relationship with the disease. Some factors are more important than others and we need this information to provide better advice to patients and the public. The hope is that such evidence will help people make reasonable (sensible) choices (like being physically active, eating a healthy diet, etc.). It is also crucial for healthcare professionals to take decisions regarding patient care.

As a matter of fact, most of the advice practitioners of modern medicine give their patients has its basis in epidemiological studies. That’s the point of epidemiology.