Devastating pathogen-borne diseases and plagues, both viral and bacterial in nature, have affected humans since the beginning of human history. The true cause of these diseases was not understood at the time, and some people thought that diseases were a spiritual punishment. Over time, people came to realize that staying apart from afflicted persons, and disposing of the corpses and personal belongings of victims of illness, reduced their own chances of getting sick.
Epidemiologists study how diseases affect a population. An epidemic is a disease that occurs in an unusually high number of individuals in a population at the same time. A pandemic is a widespread, usually worldwide, epidemic. An endemic disease is a disease that is constantly present, usually at low incidence, in a population.