Acquired immunodeficiency syndrome is an opportunistic infection and Kaposi's sarcoma results from secondary immune stimulation.

Two hypotheses are presented to explain the recent outbreak of acquired immunodeficiency syndrome (AIDS) and Kaposi's sarcoma in certain risk groups, particularly homosexuals, and the absence of these diseases from other segments of the population. According to the first, AIDS is itself an opportunistic infection. It causes disease only in individuals who are already immunocompromised by hepatitis B, cytomegalovirus, parasites, or other immunosuppressive factors. This hypothesis predicts that evidence of previous contact with the AIDS agent would be found in healthy individuals who have had the immune capability to suppress its pathogenicity. It also suggests that individuals with evidence of immunodeficiency are at risk of this syndrome and should adopt a lifestyle favouring recovery of their immune system. According to the second hypothesis, the high incidence of Kaposi's sarcoma observed in immunocompromised AIDS patients is not due to decreased immunosurveillance but to a reactive secretion of immunomodulating factors with angiogenesis-promoting activity. They are released in an attempt to bring the immune system back into balance. These factors would enhance capillary proliferation and subsequent transformation of endothelial cells, perhaps via cytomegalovirus.