epidemic

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.