A contact is a person or animal that has been in association with a disease-infected person or animal in such a way as to have had an opportunity to acquire the infection and be capable of transmitting it. A contact may not be ill or have any overt signs or symptoms of infection, yet may nevertheless transmit the infection to others who are susceptible to the infection. The word also describes the mode of transmission of infection, which may be either by direct (person-to-person) or indirect contact. Indirect contact includes transmission of infection via contaminated clothing or utensils, through an intermediary living creature such as a rat or domestic animal, or by insect vectors in which the infectious pathogen passes part of its life cycle.
Symptomless, and apparently healthy, infected individuals are a very important category of contacts. Many sexually transmitted diseases, many food-borne gastrointestinal infections, and several of the common infectious diseases are spread by apparently healthy contacts. Contact tracing is therefore an essential part of the process of surveillance and control of communicable diseases, especially sexually transmitted diseases. Quarantine was once a common method of restricting the movements of known contacts of contagious diseases such as diphtheria, and of diseases erroneously believed to be contagious, notably poliomyelitis.
JOHN M. LAST