Kaposi sarcoma (KS) is a vascular neoplastic disorder that involves spindle cell proliferation, characterized by red-purple or blue-brown macules, plaques, and nodules of the skin and other organs. Skin lesions may be firm or compressible, solitary or multiple. First described in 1872, it was considered a rare tumor of unknown etiology before its frequent diagnosis in HIV-infected patients.

There are 4 epidemiological forms of KS. The classical form occurs in older males of mainly Mediterranean or eastern European Jewish backgrounds. An endemic form occurs in all age groups in parts of equatorial Africa; neither has a known precipitating environmental factor nor is associated with immune deficiency. The remaining types—in recipients of organ transplants who undergo immunosuppressive treatment or in HIV-infected persons—are accompanied by immune impairment. Overall, males are predominantly afflicted. The epidemic form presents the most aggressive clinical course and is seen almost exclusively in HIV-infected individuals. Despite differences in clinical manifestations and serostatus, it is appropriate to consider all forms of Kaposi sarcoma as one entity, given the identical immunohistochemical features of the characteristic spindle cell of the tumor.

Kaposi sarcoma-associated herpesvirus (KSHV), or human herpesvirus 8 (HHV-8), is believed to be the causal agent of Kaposi sarcoma. Discovered in 1994, it is a new human Gammaherpesvirus related to an oncogenic herpesvirus of monkeys, Herpesvirus saimiri. Evidence of viral infection is found in virtually all cases, and several lines of evidence point to a key etiologic role in this disease. KSHV infection precedes clinical sarcoma, is highly associated with increased risk in all populations studied thus far, and affects the endothelial (spindle) cell thought to be the prime determinant of tumorigenesis. KSHV has also been shown to induce transformation of primary endothelial cells.

Sero-epidemiological analysis suggests that KSHV has a more limited distribution than any of the other 7 human herpesviruses. In North America, seroprevalence ranges from 0%–1% in blood donors to about 35% in HIV-infected individuals and up to 100% in Kaposi sarcoma patients with AIDS. In Milan, Italy, blood donors have a 4% seropositivity rate. Data suggest even higher KSHV rates in central Africa, where 58% of persons aged 14–84 were KSHV positive in one study and seroprevalence (similar in men and women) increased linearly with age.

Serological analyses also suggest that infection occurs primarily in sexually active people, particularly men who have sex with men. Differences in risk of Kaposi sarcoma for AIDS patients who acquired HIV via sexual transmission and those whose HIV infections derived from blood product exposure support the role of sexual transmission: only 1% to 3% of hemophilia- and transfusion-related AIDS patients develop Kaposi. Transplacental transmission of anti-KSHV antibody is almost certain and the virus may also be transmitted transplacentally since children of KSHV-positive mothers are at increased risk of infection after the neonatal period. In Africa, the high seroprevalence among adolescents and the relatively linear increase in prevalence with age suggest that nonsexual modes of transmission for KSHV may also be important.

There is no known cure for Kaposi sarcoma, but partial and complete remissions have been noted. Cases should be reported to a tumor registry.