Epidemiology, the study of epidemics, is sometimes called the medical aspect of ecology because it is the study of diseases in animal populations, including humans. The epidemiologist is concerned with the interactions of organisms and their environments as related to the presence of disease. Environmental factors of disease include geographical features, climate, and concentration of pathogens in soil and water. Epidemiology determines the numbers of individuals affected by a disease, the environmental circumstances under which the disease may occur, the causative agents, and the transmission of disease.

Epidemiology is commonly thought to be limited to the study of infectious diseases, but that is only one aspect of the medical specialty. The epidemiology of the environment and lifestyles has been studied since Hippocrates's time. More recently, scientists have broadened the worldwide scope of epidemiology to studies of violence, of heart disease due to lifestyle choices, and to the spread...