Carrier of Infection

(more accurately, carrier of the infectious agent; also carrier of bacteria), a human being who is infected with and thus can transmit the causative agent of an infectious disease in the absence of visible symptoms of that disease. The condition of a carrier is known as the carrier state.

The existence of a carrier state in many infectious diseases, including typhoid fever, epidemic hepatitis, and diphtheria, has been established. A carrier state has not been observed in measles, smallpox, glanders, and other infectious diseases. The carrier state may be chronic or acute. Its mechanism of action has not been sufficiently elucidated. A prolonged carrier state is often maintained by accompanying diseases. For example, angina and tonsillitis can maintain the carrier state in infections of the upper respiratory tract, and colitises, cholecystitides, and helminthiases, the carrier state in intestinal infections.

Incubatory carrier states are usually temporary and occur in individuals with no previous history of the disease. Included in diseases that are transmissible by incubatory carriers are cerebrospinal meningitis, scarlet fever, diphtheria, poliomyelitis, and cholera. Convalescent carriers have been observed to transmit cholera, typhoid and paratyphoid fevers, dysentery, diphtheria, scarlet fever, meningitis, and poliomyelitis. The convalescent carrier state rarely persists longer than a month after recovery from the disease; however, in a few diseases, including typhoid fever, the carrier state may become chronic and last for many years. Casual carriers are immune to the disease whose infectious agents they carry. Carriers are identified by laboratory isolation of the causative agent.

A carrier excretes fewer infectious agents than do patients who actually manifest the disease. However, a carrier is a source of infection and thus, depending on his occupation, can represent a serious epidemic threat. The carrier of an intestinal infectious agent is especially dangerous if he works in a food-processing or public catering enterprise, and the carrier of the diphtheria microbe, if he works around children. Dwelling conditions and personal habits are among the other factors that affect the seriousness of the threat posed by a carrier.