epidemic A process whose beginning is demarcated by accelerating infection acquisition by one or more host species of a particular pathogen in a particular location and whose end is demarcated by the time when the rate of infection acquisition becomes constant. Epidemic A process whose beginning is demarcated by accelerating infection acquisition by one or more host species of a particular pathogen in a particular location and whose end is demarcated by the time when the rate of of infection acquisition becomes constant. The constant rate may be zero. The period of an epidemic may be defined using statistical criteria based on incidence rates or causal models based on their fit to incidence data. An example of multiple host species is foot and mouth disease (such as the 2001 epidemic in the UK, see: http://www.sciencemag.org/content/292/5519/1155.long) and swine flu. The foot and mouth disease epidemic affected sheep, pigs, and cattle. William R. Hogan epidemic of infection infection in ecosystem