Guest post by MARZIEH GHIASI

MSU SciComm blog contest winner

I’ve always loved detective stories—mostly the classical figures operating out of London: Hercule Poirot, Sherlock Holmes etc. But detectives aren’t only in fiction. John Snow, a founder of epidemiology, was a real-life disease detective. Notably not the one with dragons, Snow trawled through the streets of mid-19th century London trying to trace the origins of a deadly cholera pandemic that was killing hundreds. Using clever methods, including interviews, mapping and even an early epidemiologic experiment, he isolated the outbreak to some water supply companies providing sewage-contaminated waters. One major contaminated source was a water pump, which Snow helped shut down. The pump itself, no longer in use, is still there to see on Broad Street in London.

John Snow helped bring the cholera outbreak in London under control decades before people understood that bacteria caused cholera and other disease. That has been the power of epidemiology for a century and a half: to understand patterns of health and disease and save lives even if we don’t understand the biology of a disease perfectly. However, since its 19th century origins, the field has come to encompass more than infectious diseases.


When I began my training in epidemiology, two things became quickly apparent. First, no one including my family could actually pronounce what I am doing, or they confused it with dermatology or entomology. Unfortunately, I don’t know much about the skin, nor do I have a great affinity for insects, unless they are carriers of disease. Second, while the field finds itself in headlines in times of global crisis such as the COVID-19 pandemic, few people actually know what modern epidemiologists do.

Think of COVID-19. Virologists are working around the clock to understand the mechanisms of how the SARS-CoV-2 injures cells. Clinicians are in the frontlines of healthcare reporting symptoms and treating patients. Public health officials are running campaigns to make sure everyone is informed about the need to maintain social distancing. So, what are the epidemiologists doing?

There are many complex questions of health and disease that we take for granted. Questions like ‘does taking a daily aspiring prevent heart attacks?,' ‘do warnings on cigarette packs reduce deaths from smoking?,' ‘who is at higher risk of dying from COVID-19?'—can’t exactly be solved in a laboratory because the human body is complex, and there are many variations between people, our economic and social environments.

Even in the early days, epidemiologists were looking for correlations Epidemiologists work closely with biostatisticians to understand the patterns of disease across population ‘variables’ such as age, sex, health status, social environment, economic status and so on. This has been described as a fifth dimension, where human health is measured over large populations, understood in real and imagined worlds, and described through probabilities. Similar to how meteorologists can’t predict the exact amount rain in any given backyard or on someone’s head, an epidemiologist can’t say what has happened or will happen to an individual person. But a meteorologist can, with some margin of certainty, describe average rain over a region and make a reasonable recommendation to carry umbrellas when traveling in an area. Similarly, epidemiologists can describe population health factors and make recommendations about health precautions or interventions for populations.

Today, epidemiologists in dozens of subfields are working to understand the COVID-19 pandemic. They count the number of people infected and dying (descriptive and spatial epidemiology), i