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 people 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 people 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
An epidemic disease is not required to be contagious,   and the term has been applied to West Nile fever  and the obesity epidemic (e.g. by the World Health Organisation ), among others. 
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 people affected and the number who die in successive epidemics: the severity of successive epidemics rises and falls over periods of five or ten years. 
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 (sometimes referred as mixed outbreak).
- 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.
Microsoft founder and philanthropist Bill Gates wrote an essay published in the New England Journal of Medicine in March 2015. In the essay, Gates states that an epidemic caused by a natural disaster or bioterrorism is the most likely event that could kill 10 million or more people all at the same time. 
Gates and other experts suggest that the best way to prepare for an epidemic is to have a disease surveillance system, be able to quickly dispatch emergency workers, especially local-based emergency workers, and have a legitimate way to guarantee the safety and health of health workers. 
Effective preparations for a response to a pandemic are multi-layered. The first layer is a disease surveillance system. Tanzania, for example, runs a national lab that runs testing for 200 health sites and tracks the spread of infectious diseases. The next layer is the actual response to an emergency. According to U.S.-based columnist Michael Gerson, only the U.S. military and NATO have the global capability to respond to such an emergency. 
Gates proposed that the world responded slowly to the Ebola virus outbreak because of a lack of preparation. Two weeks after the 2013 typhoon hit the Philippines, over 150 foreign medical teams were on the ground helping with injured victims. After the 2005 earthquake in Pakistan, a team of aid workers who had been helping Afghan refugees for several decades were able to get to the victims in less than 24 hours. Dr. Bruce Aylward, assistant director general for emergencies at the World Health Organization, says that in the case of the Ebola outbreak, "there was no way anyone could guarantee the right of medical evacuation for people affected by Ebola.” Aylward said that for future emergency health workforces, it is critical to offer a "duty of care", meaning the ability to ensure that aid workers will be taken care of in terms of health and safety. 
The CDC will scale back global disease prevention efforts by 80 percent by October 2019 due to a lack of funding (as funding had been temporarily increased in 2014 to address the Ebola epidemic).  Bill Gates has warned that epidemic disease will kill more than 30 million people in the next 10-15 years. 
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, Third Edition (PDF). Atlanta, Georgia: Centers for Disease Control and Prevention. 2012.
- Green MS; Swartz T; Mayshar E; Lev B; Leventhal A; Slater PE; Shemer Js (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" (PDF). Emerging Infect. Dis. 12 (6): 976–80. doi: 10.3201/eid1206.051263. PMC 3373038. PMID 16707055.
- Controlling the global obesity epidemic, the World Health Organisation
- "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.
- Gerson, Michael (30 March 2015). "The next epidemic". The Greenville News. South Carolina, US.
- Silver, Marc (30 March 2015). "Bill Gates Tells The World: Get Ready For The Next Epidemic". NPR. Retrieved 1 April 2015.
- Thomsen, Jacqueline (2 February 2018). "CDC to cut global disease prevention efforts by 80 percent".
- "Bill Gates thinks an infectious disease outbreak could kill 30 million people in the next decade — but the US is cutting efforts to prevent global pandemics".
- 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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- European Centre for Disease Prevention and Control
- International Epidemiological Association (IEA)
- A Dictionary of Epidemiology (IEA)
- People's Epidemiology Library
- Video Discussion of the Prostate Cancer Epidemic
- Monash Virtual Laboratory - Simulations of epidemic spread across a landscape
- "The Next Epidemic — Lessons from Ebola", essay by Bill Gates, New England Journal of Medicine (March 18, 2015)