HUMAN IMMUNODEFICIENCY VIRUS
HIV is the causative agent of AIDS. The definition of AIDS, as determined by the CDC, has changed several times since the syn-drome was first recognized in 1981. In general, the definition sets a point in the continuum of HIV pathogenesis in which the host has clinically demonstrated profound immune dysfunction. Many opportunistic infections and neoplasms serve as markersfor immunosuppression severity. Since 1993, the AIDS defini-tion has also included a CD4-positive (CD4+) cell count of less than 200 as a threshold criterion. CD4+ cells are a subset of lymphocytes and one of the targets of HIV infection.
HIV is transmitted through sexual contact, percutaneous in-jection of contaminated blood, or perinatally from infected mother to fetus. Most people infected by the percutaneous route are intravenous or injecting drug users who share contaminated needles, but transmission is also remotely possible through con-taminated blood transfusion. Since 1985, all blood transfusions have been screened, and transfusion-related transmission of HIV is now extremely unlikely.
Health care workers can be infected through the percutaneous route if needlestick or other injury from a sharp object introduces contaminated blood. Prospective studies of this risk demonstrate that less than 1% of such occupational exposures (in which the source patient is infected with HIV) lead to transmission (CDC 2001a). Despite the rarity of transmission, health care workers are advised to take extreme care to avoid needlestick or mucous membrane exposure to blood of all patients. Since 2001, employers in health care institutions are required to provide devices designed to reduce the risk of needlestick and other injury when such devices are found to be beneficial. Since 1996, the CDC has recommended postexposure prophylaxis for significant occupa-tional exposures to HIV. Counseling about the advisability of prophylaxis and appropriate medication and dose selection should be made on a case-by-case basis. Table 70-5 provides the recommended algorithm to determine which combination of antiretroviral drugs should be offered to the exposed health care worker. All health care workers should understand the need to re-port a needlestick or other percutaneous exposure immediately (CDC, 2001f ).