This is a method that has been used in the control of endemic contagious disease for decades. [Hethcote HW, Yorke JA. Gonorrhea Transmission Dynamics and Control. New York: Springer-Verlag; 1984.] A disease investigation begins when an individual is identified as having a communicable disease. An investigator interviews the patient, family members, physicians, nurses, and anyone else who may have knowledge of the primary patient’s contacts, anyone who might have been exposed, and anyone who might have been the source of the disease. Then the contacts are screened to see if they have or have ever had the disease. The type of contact screened depends on the nature of the disease. A sexually transmitted disease will require interviewing only infected patients and screening only their sex partners. A disease that is spread by respiratory contact, such as tuberculosis, may require screening tens to hundreds of persons, such as other inmates in a prison.

Many persons object to contact tracing as an invasion of privacy. Since contact tracing is constitutionally permissible, these objections are often disguised as criticisms of the cost of contact tracing. Contact tracing is an expensive process but one that is cost- effective because it is highly efficient in finding infected persons. [Potterat JJ, Spencer NE, Woodhouse DE, Muth JB. Partner notification in the control of human immunodeficiency virus infection. Am J Public Health. 1989 July;79:874.] This was best demonstrated in the campaign to eradicate smallpox.

Contrary to popular belief, smallpox was not controlled by immunizing every person on earth. It was controlled by extensive contact tracing to find infected individuals. Smallpox could be controlled only because the sores and scars prevented infected persons from escaping detection. [Carrell S, Zoler ML. Defiant diseases: hard-won gains erode. Med World News. 1990;31:12–20.] Fellow villagers and tribesmen were encouraged in various ways to identify infected persons. When a person with smallpox was identified, he or she was quarantined, and all the persons in the surrounding community or village were vaccinated. In this way smallpox was eventually reduced to isolated outbreaks and then eradicated.

Although many health departments have resisted contact tracing for HIV infection, the resurgence of infectious tuberculosis secondary to HIV-induced immunosuppression is forcing them to reexamine this policy. [CDC. Transmission of multidrug-resistant tuberculosis from an HIV-positive client in a residential substance- abuse treatment facility—Michigan. MMWR. 1991;40:129.] Even gay groups traditionally opposed to HIV reporting and contact tracing are beginning to recognize its benefits. Recent outbreaks of drug-resistant tuberculosis have intensified the concern with contact tracing. Drug-resistant tuberculosis poses great public health problems because of its often fatal course and the inability to render the carriers noninfectious. [CDC. Outbreak of multidrug-resistant tuberculosis— Texas, California, and Pennsylvania. MMWR. 1990;39:369.]