Have you ever heard of an "epidemic", perhaps as in a "flu epidemic", or "polio epidemic"? "Epidemiology" is a branch of medicine that deals with the study of "epidemic" diseases. "Epidemics" usually affect a lot of people at one time or in one place. So "epidemiology" can include studying groups of people who have been exposed to something in the environment (like pesticides, or ozone) to see if they have been affected by this exposure.
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