People with all types of Kaposi's sarcoma tumors are infected with a virus called human herpesvirus 8 (HHV-8), or Kaposi's sarcoma-associated herpesvirus (KSHV). Infection with this virus is necessary for Kaposi's sarcoma to develop -- but not everyone who is infected with the virus develops Kaposi's sarcoma.
Exactly how HHV-8 is passed from one person to another is not completely understood. Men who have sex with men are more likely to develop Kaposi's sarcoma than other people with HIV infection. People who have been infected with HIV through intravenous drug use or heterosexual sex also have an increased risk of developing Kaposi's sarcoma. The virus may also pass from person to person through other routes.
Immunodeficiency is an important risk factor for Kaposi's sarcoma. Even before the AIDS epidemic, doctors noted that Kaposi's sarcoma tended to occur more often in patients with weakened immune systems such as those who took immunosuppressive drugs to prevent the rejection of a transplanted organ. Up to 5 percent of kidney transplant recipients have been reported to develop Kaposi's sarcoma.
Most forms of Kaposi's sarcoma are more common in men than in women, but the reason for this difference is not known.
