Example of an<unk>  showing the number of new infections over time. An<unk>  (from Greek ἐπί epi "upon or above" and δῆμος demos "people") is the rapid spread of disease to a large number of hosts in a given population within a short period of time. For example, in meningococcal infections, an attack rate in excess of 15 cases per 100,000 people for two consecutive weeks is considered an<unk> .<unk> s of infectious disease are generally caused by several factors including a change in the ecology of the host population (e.g., increased stress or increase in the density of a vector species), a genetic change in the pathogen reservoir or the introduction of an emerging pathogen to a host population (by movement of pathogen or host). Generally, an<unk>  occurs when host immunity to either an established pathogen or newly emerging novel pathogen is suddenly reduced below that found in the endemic equilibrium and the transmission threshold is exceeded. An<unk>  may be restricted to one location; however, if it spreads to other countries or continents and affects a substantial number of people, it may be termed a pandemic. The declaration of an<unk>  usually requires a good understanding of a baseline rate of incidence;<unk> s for certain diseases, such as influenza, are defined as reaching some defined increase in incidence above this baseline. A few cases of a very rare disease may be classified as an<unk> , while many cases of a common disease (such as the common cold) would not. An<unk>  can cause enormous damage through financial and economic losses in addition to impaired health and loss of life.
Epidemic
Epizootic
Endemic 
Pandemic
Toxoid
Acute 
Pre-medical
Infodemic