A rare type of cancer characterized by the abnormal growth of cells that line lymph and blood vessels. Kaposi sarcoma (KS) causes red or purple patches of tissue (lesions) to grow under the skin and in the lining of the mouth, nose, and throat. Lesions may also develop in the digestive tract, liver, or lungs. KS generally occurs in people with weakened immune systems. In people with HIV, KS is an AIDS-defining condition.