HIV is the cause of acquired immunodeficiency syndrome (AIDS), a slowly debilitating, almost always fatal, human disease. The disease has been recognized relatively recently as a specific entity, and in 1985 criteria for the diagnosis of AIDS were developed (cf. Purtilo et al., 1986). Since then, the disease has increased dramatically in numbers throughout the world. As of mid-1993, the estimated distribution of HIV infections in adults throughout the world is shown in Figure 12.24 (Merson, 1993). Although there is no definitive evidence that the HIV virus actu- ally transforms cells to neoplasia unaided by other viruses, the IARC has stated that infection with HIV-1 is carcinogenic to humans, while infection with HIV-2 is possibly carcinogenic to humans (IARC, 1996). We have already noted above the dramatic increase in Kaposi sarcoma seen in AIDS patients and resulting from infection and transformation of cells with HHV-8 (see above). A listing of neoplasms developing in AIDS patients at levels significantly higher than those of the average population is seen in Table 12.10. In general, “non-AIDS” cancers exhibit an increased risk of 1.9 in a large series of AIDS patients recently studied (Goedert et al., 1998). The vast majority of AIDS-related lymphomas are associated with EBV infections (Gaidano and Dalla-Favera, 1995). Increased incidences of many of the other neoplasms seen in the table may be related to infection with other potentially oncogenic herpesviruses or, more likely, with the dramatically compromised immune system, allowing the growth of neoplasms that otherwise might have been suppressed by a normal immune response (Chapter 18).