Kaposi s sarcoma (KS) is caused by a gammaherpesvirus called Kaposi s sarcoma-associated herpesvirus (KSHV), or human herpesvirus-8 (HHV-8). However, infection with KSHV is not sufficient to cause KS, and HIV infection is an important cofactor. Treatment of HIV with potent antiretroviral therapy can reduce the risk of KS, and can also induce regression in patients with established HIV-KS. One mechanism by which HIV is believed to contribute to KS is through HIV-induced immunodeficiency which leads to a loss of immunologic control of KSHV and/or KS itself. However, other mechanisms may also contribute.