Kaposi's sarcoma 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. The red and purple patches often cause no symptoms, though they may be painful. If the cancer spreads to the digestive tract or lungs, bleeding can result. Lung tumors can make breathing hard.
Before the HIV/AIDS epidemic, KS usually developed slowly. In HIV/AIDS patients, though, the disease moves quickly. Treatment depends on where the lesions are and how bad they are. Treatment for HIV itself can shrink the lesions. However, treating KS does not improve survival from HIV/AIDS itself.
NIH: National Cancer Institute