The Plague of Athens (c. 1652–1654) by Michiel Sweerts, illustrating the devastating<unk>  that struck Athens in 430 BC, as described by the historian Thucydides There are several changes that may occur in an infectious agent that may trigger an<unk> . These include: * Increased virulence * Introduction into a novel setting * Changes in host susceptibility to the infectious agent An<unk>  disease is not required to be contagious, and the term has been applied to West Nile fever and the obesity<unk>  (e.g., by the World Health Organization), among others. The conditions which govern the outbreak of<unk> s include infected food supplies such as contaminated drinking water and the migration of populations of certain animals, such as rats or mosquitoes, which can act as disease vectors.<unk> s can be related to seasonality of certain infectious agents. Seasonality may enter into any of the eight key elements of the system: (1) susceptible recruitment via reproduction, (2) transmission, (3) acquired immunity and recovery, (4) waning immunity, (5) natural mortality, (6) symptomatology and pathology (which may be acute or chronic, depending on the disease), (7) disease-induced mortality, and (8) cross-species transmission. Influenza, the common cold, and other infections of the upper respiratory tract, such as sore throat, occur predominantly in the winter. There is another variation, both as regards the number of people affected and the number who die in successive<unk> s: the severity of successive<unk> s rises and falls over periods of five or ten years.
Epidemic
Epizootic
Endemic 
Pandemic
Toxoid
Acute 
Pre-medical
Infodemic