Human immunodeficiency virus (HIV) is the cause of AIDS, an illness that interferes with the body's ability to fight infection and some types of cancer. Human immunodeficiency virus passes from person to person through contact with blood or other body fluids that contain the virus. Before treatments were available, most people infected with HIV developed, and often died of, infections or certain types of cancer that meant they had AIDS (known as “AIDS-defining cancer”). Kaposi sarcoma, non-Hodgkin lymphoma, and advanced cervical cancer are examples of AIDS-defining cancer.