What scientist studies the spread of diseases?

The scientist who studies the spread of diseases is an epidemiologist.

Epidemiologists investigate patterns of illness in populations rather than treating individual patients. They ask who is affected, where and when cases occur, how a disease may be transmitted, and which factors increase risk. During an outbreak, they may interview patients, construct timelines, compare exposed and unexposed groups, and use statistical models to identify likely sources.

The field is broader than infectious disease. Epidemiologists also study cancer, heart disease, injuries, environmental exposures and health-care outcomes. Their findings guide vaccination campaigns, food-safety investigations, screening recommendations and public-health policy. A clinician usually focuses on diagnosing and treating one patient; an epidemiologist focuses on patterns and prevention across groups of people.