Kaposi's sarcoma. Skin nodules on an AIDS patient's right hand and lower arm caused by Kaposi's sarcoma. 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 is a relatively rare malignant skin tumour, which results in purple-coloured plaques that eventually form nodules. In AIDS it often spreads to other tissues and can be rapidly fatal.