Epidemic
Epidemic
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 (such as an infected food server), animal (for example, a 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, influenza, 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, as 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 can spread 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
KEY TERMS
Epidemic curve— A graph showing the number of cases of an outbreak of illness by the date of illness onset.
Herd immunity— Overall protection of a population from a disease by immunization of most of the population.
Zoonoses— Diseases spread by animals to humans.
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.
An epidemic that erupts in a wide distribution of disease and usually crosses international and geographical borders is termed a pandemic. Examples of pandemics in modern history include AIDS and the worldwide influenza pandemic of 1918.
See also Epidemiology; Severe acute respiratory syndrome (SARS); Tropical diseases.
Resources
BOOKS
Crosby, Alfred W. America’s Forgotten Pandemic: The Influenza of 1918. Cambridge: Cambridge University, 2003.
PERIODICALS
Phua, Kai-Lit, and Lee, Lai Kah. “Meeting the Challenge of Epidemic Infectious Disease Outbreaks: An Agenda for Research.” Journal of Public Health Policy. 26 (2005): 122–132.
OTHER
World Health Organization “Global Outbreak & Alert Response Network.” <http://www.who.int/csr/outbreaknetwork/en/> (accessed November 26, 2006).
Marc Kusinitz
Epidemic
Epidemic
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
epidemic
Epidemics come with wings and slowly limp away (from a French proverb).
The word ‘epidemic’ has an emotional ring to it. This is probably the reason why it is often used wrongly when, strictly speaking, other epidemiological terms such as pandemics or outbreaks should be employed to enumerate disease. An epidemic (from the Greek: epi upon; demos, people) is usually defined as a large-scale, temporary increase in the occurrence of a disease in a community or region which is clearly in excess of normal expectancy, whereas a pandemic (pan, all) is the occurrence of a disease which is clearly in excess of normal expectancy and is spread over a whole geographical area, usually crossing national boundaries.
At the other end of the scale are outbreaks and sporadic cases. An outbreak may be a household or a general outbreak. A household outbreak involves two or more persons resident in the same household whose illness is associated in time but not apparently connected with any other case or outbreak. A general outbreak involves two or more persons who are not confined to one household but are associated in time and location. A sporadic case refers to a person whose illness is not apparently connected with similar illnesses in any other persons.
A disease or infectious agent is said to be endemic when it is constantly present within a given location or population group.
Although the term ‘epidemic’ is used widely to describe clusters of disease in general, and even in a non-medical sense (e.g. an epidemic of road rage), it has traditionally been used when infection strikes a population. This often occurs when there is crowding together of humans (or, for that matter, animals, fish, or birds), as this provides the necessary conditions to allow microorganisms to multiply and spread. When humans led nomadic lives there was often less chance for epidemics to occur; the main opportunities came when large numbers gathered for such things as pilgrimages or wars — and when subsequently the group dispersed the chances of carrying the infection elsewhere were multiplied.
The threat of epidemics in overcrowded and difficult conditions is particularly well illustrated in military history; on many occasions the germ has been as important as the sword or gun in determining the outcome of a campaign. The Spanish conquest of Mexico owes much of its success to an epidemic of smallpox that destroyed about half of the Aztec population. The typhoid bacillus caused severe effects during both the American Civil War (1861–5) and the Boer War (1899–1902). The use of typhoid vaccine in the latter years of World War I meant that the main impact of typhoid in this war subsided after 1916. Similarly, typhus was rife in the Civil War in Britain (1642–9), when both the Parliamentary and Royalist armies were affected.
There are three main patterns of epidemic, determined by the mode of transmission of the microorganism.
Firstly, the explosive epidemic. This is characterized by the occurrence of many cases in a relatively short period; there is a sharp rise and fall in the number of infected persons, since the usual cause of such an event is a common source of infection. This type of epidemic is thus also frequently termed a common source epidemic or a point source epidemic. This pattern of infection often occurs when water or food becomes contaminated, although other vehicles of infection can also be responsible.
Secondly, person-to-person spread. These epidemics usually have a more protracted course, taking longer than explosive epidemics to build up and subside. An infective agent may be passed from person to person by a variety of routes (e.g. respiratory or gastrointestinal). Diseases such as influenza or chickenpox often follow this pattern.
Thirdly — a combination of the two — an explosive epidemic with subsequent person-to-person spread. This pattern is apparent when there is contamination of a common water or food source and the initial cases then infect their contacts. Although this type of epidemic starts in the same way as an explosive incident, there is a slower decline.
The importance of keen observation and recording of epidemics in order to deduce the likely cause has been demonstrated on many occasions, and may be considerably in advance of microbiological proof. In 1849, 34 years before the identification of Vibrio cholerae by Robert Koch (1843–1910), John Snow (1813–58), a London physician, proved by epidemiological observation that cholera is mainly spread by drinking infected water, rather than through the air in the form of a miasma as was commonly thought at the time. Similarly, William Budd (1811–80), a general practitioner in Devon, showed in 1873 how typhoid was caused, even though it was not until 1885 that Salmonella typhi was first isolated in the laboratory. More recently, William Pickles (1885–1969), a general practitioner in Wensleydale, Yorkshire, was able to elucidate many of the epidemiological characteristics of viral hepatitis well before microbiological advances were to confirm his observations.
Daniel Reid
Bibliography
Pickles W. N. (1939). Epidemiology in country practice. Wright, Bristol. (Re-issued in 1972 by the Devonshire Press, Torquay.)
Tyrrel, D. A. J. (1982). The abolition of infection. Hope or illusion? Rock Carling Fellowship Lecture. Nuffield Provincial Hospitals Trust, London.
See also infectious diseases; pandemics.
epidemic
ep·i·dem·ic / ˌepiˈdemik/ • n. a widespread occurrence of an infectious disease in a community at a particular time: a flu epidemic. ∎ a disease occurring in such a way. ∎ a sudden, widespread occurrence of a particular undesirable phenomenon: an epidemic of violent crime.• adj. of, relating to, or of the nature of an epidemic: shoplifting has reached epidemic proportions.