The study of patterns and possible causes of diseases in humans.

Epidemiologists study short-term outbreaks of disease, called epidemics, and long-term diseases such as cancer and heart disease. The challenge in epidemiology is to determine whether a true relationship exists between the cause being studied and the resultant disease. Epidemiologists must assess possible effects of factors other than those specifically being studied (called "confounders"). Epidemiologists seek "statistically significant" evidence that a relationship exists between the factor being studied and the incidence of disease. To be "statistically significant" the relationship must be due to more than just chance.