An epidemic (from Greek ἐπί epi "upon or above" and δῆμος demos "people") is the rapid spread of infectious disease to a large number of persons in a given population within a short period of time, usually two weeks or less. For example, in meningococcal infections, an attack rate in excess of 15 cases per 100,000 persons for two consecutive weeks is considered an epidemic. 
Epidemics 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 epidemic 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 epidemic 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 epidemic usually requires a good understanding of a baseline rate of incidence; epidemics 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 epidemic, while many cases of a common disease (such as the common cold) would not.
- Increased virulence
- Introduction into a novel setting
- Changes in host susceptibility to the infectious agent
The conditions which govern the outbreak of epidemics 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. Certain epidemics occur at certain seasons: for example, whooping-cough occurs in spring, whereas measles produces two epidemics, one in winter and one in March. 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 persons affected and the number who die in successive epidemics: the severity of successive epidemics rises and falls over periods of five or ten years.
Common source outbreak
In a common source outbreak, the affected individuals had an exposure to a common agent. If the exposure is singular and all of the affected individuals develop the disease over a single exposure and incubation course, it can be termed a point source outbreak. If the exposure was continuous or variable, it can be termed a continuous outbreak or intermittent outbreak, respectively.:56
In a propagated outbreak, the disease spreads person-to-person. Affected individuals may become independent reservoirs leading to further exposures.:56
Many epidemics will have characteristics of both common source and propagated outbreaks. For example, secondary person-to-person spread may occur after a common source exposure or an environmental vectors may spread a zoonotic diseases agent.:56–58
- Airborne transmission: Airborne transmission is the spread of infection by droplet nuclei or dust in the air. Without the intervention of winds or drafts the distance over which airborne infection takes place is short, say 10 to 20 feet.
- Arthropod transmission: Arthropod transmission takes place by an insect, either mechanically through a contaminated proboscis or feet, or biologically when there is growth or replication of an organism in the arthropod.
- Biological transmission: Involving a biological process, e.g. passing a stage of development of the infecting agent in an intermediate host. Opposite to mechanical transmission.
- Colostral transmission: A form of vertical transmission via successive generations.
- Contact transmission: The disease agent is transferred directly by biting, sucking, chewing or indirectly by inhalation of droplets, drinking of contaminated water, traveling in contaminated vehicles.
- Cyclopropagative transmission: The agent undergoes both development and multiplication in the transmitting vehicle.
- Developmental transmission: The agent undergoes some development in the transmission vehicle.
- Fecal-oral transmission: The infectious agent is shed by the infected host in feces and acquired by the susceptible host through ingestion of contaminated material.
- Horizontal transmission: Lateral spread to others in the same group and at the same time; spread to contemporaries.
- Mechanical transmission: The transmitter is not infected in that tissues are not invaded and the agent does not multiply.
- Propagative transmission: The agent multiplies in the transmission vehicle.
- Vertical transmission: From one generation to the next, perhaps transovarially or by intrauterine infection of the fetus. Some retroviruses are transmitted in the germ line, i.e. their genetic material is integrated into the DNA of either the ovum or sperm.
The term epidemic derives from a word form attributed to Homer's Odyssey, which later took its medical meaning from the Epidemics, a treatise by Hippocrates. Before Hippocrates, epidemios, epidemeo, epidamos, and other variants had meanings similar to the current definitions of "indigenous" or "endemic." Thucydides' description of the Plague of Athens is considered one of the earliest accounts of a disease epidemic. By the early 17th century, the terms endemic and epidemic referred to contrasting conditions of population-level disease, with the endemic condition at low rates of occurrence and the epidemic condition widespread.
- List of epidemics
- Epidemic model
- Endemic (epidemiology)
- Attack rate
- Epi Info
- European Centre for Disease Prevention and Control
- Principles of Epidemiology, Second Edition. Atlanta, Georgia: Centers for Disease Control and Prevention.
- Green MS, Swartz T, Mayshar E, Lev B, Leventhal A, Slater PE, Shemer J (January 2002). "When is an epidemic an epidemic?". Isr. Med. Assoc. J. 4 (1): 3–6. PMID 11802306.
- "epidemic". The Encyclopedia of Ecology and Environmental Management, Blackwell Science. Oxford: Blackwell Publishers, 1998. Credo Reference. Web. 17 September 2012.
- Martin PM, Martin-Granel E (June 2006). "2,500-year evolution of the term epidemic". Emerging Infect. Dis. 12 (6): 976–80. doi:10.3201/eid1206.051263. PMC 3373038. PMID 16707055.
- "Epidemic." Black's Medical Dictionary, 42nd Edition. London: A&C Black, 201e. Web. 17 September 2012.
- "transmission." Saunders Comprehensive Veterinary Dictionary. Philadelphia: Elsevier Health Sciences, 2007. Credo Reference. Web. 17 September 2012.
- Lodge, T. (1603). A treatise of the plague: containing the nature, signes, and accidents of the same, with the certaine and absolute cure of the fevers, botches and carbuncles that raigne in these times. London: Edward White.
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