Kaposi's sarcoma skin plaques on the skin of an AIDS patient. AIDS (acquired immune deficiency syndrome) is a disease caused by the human immunodeficiency virus (HIV). It causes a gradual weakening of the immune system, making otherwise harmless infections life-threatening. Kaposi's sarcoma (KS) is a cancer that causes patches of abnormal tissue to grow under the skin, in the lining of the mouth, nose, and throat or in other organs. The patches are usually red or purple and are made of cancer cells and blood cells. KS is now believed to be caused by a virus called human herpes virus 8 (HHV-8). In immunosuppressed patients HHV-8 is thought to encourage normal cells to change into tumour cells.