Kaposi's sarcoma. Light micrograph of a section through part of the lung of an AIDS patient, showing a lesion caused by Kaposi's sarcoma. An alveolus (air sac, left) is filled with stained macrophages, a type of white blood cell. 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 type of skin cancer. In AIDS it often spreads to other tissues and can be rapidly fatal. For a lower power image of the same lesion see M112/322.