Over the course of the coronavirus pandemic, immunologists, virologists, and epidemiologists have all appeared in the news to help explain what’s going on and how we should respond. And when someone’s identified as one of those experts, you can often take that to mean “This person knows what they’re talking about.”

But while all three professions have made vital contributions to the fight against COVID-19—and there is some collaboration when it comes to dealing with a pandemic—they’re far from interchangeable. The best way to tell them apart is to focus on the root word of each title.

Immunologists, for one, deal with immune systems. “We study how the immune system operates in normal health and how it contributes to disease,” Dr. Steven Bensinger, an immunologist at the University of California, Los Angeles, explained in 2018. An immunologist might stick to researching how the immune system behaves when threatened, or they might delve into the clinical side: treating patients who have autoimmune diseases or allergies in which their immune systems aren’t functioning as they should be.

Virologists, meanwhile, study the viruses themselves—their structure, how they replicate, what diseases they cause, how to classify them, and so on. Understanding how viral pathogens behave in our bodies and being able to identify similarities between new and old viruses are both important aspects of learning how to fight them. And since vaccines involve teaching our immune systems how to neutralize a virus, immunologists and virologists may both help in the development of new vaccines.

Epidemiologists, on the other hand, focus more on what’s going on outside the body. In the CDC’s words, “Epidemiology is 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 problems.” Basically, epidemiologists track the spread of a disease, which includes investigating how it gets transmitted, identifying which demographics it affects, and advising people on what safety measures to take to keep from catching it (among other things). In other words, epidemiologists study epidemics.

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