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An epidemic is an outbreak of a disease among members of a specific population that exceeds the extent of occurrence of the disease normally found in that population. Epidemics affect those members of the population who do not have an acquired or inherent immunity to the disease. Although most epidemics are caused by infectious organisms, the term can be applied to an outbreak of any chronic disease, such as lung cancer or heart disease.

During an epidemic, organisms can spread in several ways. But in each case, there must be a continual source of disease organisms, that is, a reservoir of infection. The reservoir may be human (an infected food server), animal (bacteria-carrying rat), or an inanimate object (contaminated water).

For human diseases, the human body is the principal living reservoir of infectious organisms. Among the diseases spread by human carriers are AIDS, typhoid fever, hepatitis, gonorrhea, and streptococcal infections. While people who have signs and symptoms of a disease are obvious reservoirs of infections, some people may carry and spread the disease without showing any signs or symptoms. Still others may harbor the disease during the symptomless stage called the incubation period, before symptoms appear, or during the convalescent period, during which time they are recovering. This fuels the epidemic, since there is no apparent reason for people to take precautions to prevent transmission to others.

Animal reservoirs can also spread diseases. Those diseases that are spread from wild and domestic animals to humans are called zoonoses. Yellow fever, spread by the Aedes mosquito; Lyme disease, spread by ticks; rabies, spread by bats, skunks, foxes, cats, and dogs; and bubonic plague, spread by rats, are examples of zoonoses.

Inanimate reservoirs, such as drinking water contaminated by the feces of humans and other animals, are a common environment for organisms causing gastrointestinal diseases.

Once infected, a person becomes a reservoir and can spread the disease in a variety of ways, called contact transmission. Direct contact transmission occurs when an infected host has close physical contact with a susceptible person. This person-to-person transmission can occur during touching, kissing, and sexual intercourse. Viral respiratory diseases such as the common cold and influenza are transmitted in this way, as are smallpox, hepatitis A, sexually transmitted diseases (genital herpes, gonorrhea, syphilis) and, in some cases, AIDS.

Indirect contact transmission occurs when the disease-causing organism is transmitted from the reservoir to a susceptible host by means of a inanimate carrier called a fomite, which can be a towel, drinking cup, or eating utensils. Hepatitis and AIDS epidemics can occur when contaminated syringes serve as fomites among intravenous drug users.

Droplet transmission of an epidemic occurs when microbes are spread in tiny bits of mucous called droplet nuclei that travel less than three feet from the mouth and nose during coughing, sneezing, laughing, or talking. Influenza, whooping cough, and pneumonia are spread this way.

Transmission of epidemics can occur through food, water, air, and blood, among other objects. Waterborne transmission occurs through contaminated water, a common means by which epidemics of cholera, waterborne shigellosis, and leptospirosis occurs. Foodborne poisoning in the form of staphylococcal contamination may occur when food is improperly cooked, left unrefrigerated, or prepared by an infected food handler.

Airborne transmission of viral diseases such as measles and tuberculosis occurs when the infectious organisms travel more than 3 ft (0.9 m) from the human reservoir to a susceptible host in a fine spray from the mouth and nose. Fungal infections such as histoplasmosis, coccidioidomycosis, and blastomycosis can be spread by airborne transmission as their spores are transported on dust particles.

Vectors, usually insects, are animals that carry pathogens from one host to another. These vectors may spread an epidemic by mechanical or biological transmission. When flies transfer organisms causing typhoid fever from the feces of infected people to food, the disease is spread through mechanical transmission. Biological transmission occurs when an arthropod bites a host and ingests infected blood. Once inside the arthropod vector, the disease-causing organisms may reproduce in the gut, increasing the number of parasites that can be transmitted to the next host. In some cases, when the host is bitten, the parasites are passed out of the vector and into a wound when the vector passes feces or vomits. The protozoan disease malaria is spread by the Anopheles mosquito vector.

After an epidemic is introduced into a population by one or more persons, the infection spreads rapidly if there are enough susceptible hosts. If so, the incidence of the disease increases over time, until it reaches a maximum, at which time it begins to subside. This subsidence of the epidemic is due mostly to the lack of susceptible individuals; most individuals already have the disease or have had the disease and gained an immunity to it.

After an epidemic subsides, there are too few susceptible individuals to support a new epidemic if the infection is reintroduced. This overall immunity of a host population to a potential epidemic disease is called herd immunity.

Herd immunity tends to disappear over time due to three factors: (1) deterioration of individual immunity; (2) death of immune individuals; (3) influx of susceptible individuals by birth or emigration into the area of the epidemic.

The rise in the number of infected individuals over time, followed by a fall to low levels, can be graphically depicted as an "epidemic curve," which usually represents a time period of days, weeks, or months. Some may even last years. For example, waterborne gastrointestinal infections may peak during the later summer months, reflecting the role of recreational swimming in areas where parasitic organisms exist.

Marc Kusinitz

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Science EncyclopediaScience & Philosophy: Ephemeris to Evolution - Historical Background