The Control of Communicable Diseases Manual, published by the American Public Health Association, defines a carrier as a person or animal that harbors a specific infectious agent without discernable clinical disease and serves as a potential source of infection. Several varieties of carrier are recognized.
HEALTHY OR ASYMPTOMATIC CARRIERS
These are persons who harbor an infectious agent but never fall ill or manifest any overt evidence that they are infected. This commonly happens with certain virus diseases, such as several forms of viral hepatitis and poliomyelitis, and with some bacterial diseases, including diphtheria and meningococcal meningitis. The pathogenic organisms responsible for these and other diseases in this category live as commensal organisms in the carrier's respiratory and/or gastrointestinal tract, apparently coexisting without causing disease because the host is not susceptible to infection or because the organisms are not virulent. Carriers of the diphtheria bacillus, meningococcus, or other organisms in this category manifest no symptoms or signs of infection, but the organisms can be recovered from their nose, throat, or from their feces. Moreover, appropriate serological tests may reveal evidence of inapparent infection. Animals and birds can be carriers of some human diseases. For instance, jungle yellow fever is carried by monkeys and possibly other jungle-dwelling small mammals. The rabies virus, which is almost invariably fatal when humans are infected and not protected immediately by antirabies vaccine, is carried by bats who seem to suffer no ill effects at all. Salmonella organisms may be carried by many varieties of animals and birds—cattle, rodents, pets such as dogs and cats, tortoises, chickens, ducks—and of course by humans. Some may be healthy carriers, and others are chronic carriers who were infected at some time in the past and have not stopped harboring and excreting the infectious agent.
INCUBATORY AND CONVALESCENT CARRIERS
The pathogenic organisms that cause many diseases can infect healthy people who come into contact with the person harboring the pathogen either during the incubation period or during convalescence. One of the reasons that measles is so highly infectious is that it is at its most contagious stage shortly before the characteristic rash appears. Experienced family physicians and pediatricians soon learn how to recognize children in the incubation period of measles and do their best to minimize transmission by isolating children with a dry cough, red eyes, and fever. Although we should no longer have to be concerned about the risks of transmitting measles when virtually all children are immunized, occasional outbreaks still occur, so continuing vigilance is necessary. Some types of the common cold can similarly be transmitted during the incubation period.
Many diseases remain infectious for a time after the acute stage has passed and the sick person has become apparently well again. Persons in this category are convalescent carriers, and are the most frequent source of infection for others. The common cold is very often transmitted by incubatory and convalescent carriers—and, of course, by those who carry on with their normal activities without acknowledgment and treatment of their cold. Convalescent carriers can be a serious problem with many forms of diarrheal diseases, notably shigella and sonne dysentery. Investigation of outbreaks of diarrhea often reveals that the source of infection is a carrier who works as a cook or food handler in a restaurant. Among the best known, though rare, carrier states in this category, is typhoid. Convalescent cases can remain infectious for long periods, especially when typhoid bacilli lurk in such organs as the gall bladder, whence they are excreted in the feces. Typhoid carriers commonly have had overt disease at some time in the past and continue to carry typhoid bacilli, which they excrete and thereby expose others to infection. But some typhoid carriers have no history of overt disease in the past. The infamous Typhoid Mary may have been an example. The convalescent carrier state may be temporary, lasting only until residual symptoms disappear, or it may be prolonged, chronic, or even permanent.
A very important category of carrier state occurs with many sexually transmitted diseases (STDs) and with HIV (human immunodeficiency virus) infection. Persons with STDs and HIV are more often outwardly well than overtly unwell, and all too often they are sexually active. There are many recorded instances of a sexually promiscuous person with gonorrhea infecting one hundred or more people over a relatively short period.
JOHN M. LAST
Chiu, J., ed. (2000). Control of Communicable Diseases Manual, 17th edition. Washington, DC: American Public Health Association.
Last, J. M. (1997). Public Health and Human Ecology, 2nd edition. Stanford, CA: Appleton and Lange.