Also known as Kaposi's angiosarcoma, Kaposi's sarcoma is a form of skin cancer that used to be very rare in the pre-AIDS era but which has become common amongst AIDS patients with about 15-20% of AIDS patients developing this complication. In non HIV positive individuals, Kaposi's sarcoma was an unusual finding in some elderly patients (relatively more common amongst those of African or Middle East origin) and an uncommon finding in immunosuppressed patients (usually transplant patients, who have an incidence of KS about 500x more than the general population) -- though it is also known to regress spontaneously in transplant patients whose immune systems have recovered.
The lesions in Kaposi's are flat, painless and red to dark purple like a bruise on white skin (blackish on dark skin). There are actually proliferating capillary vessels and perivascular connective tissue cells. They can appear anywhere on the skin including the mouth and gastrointestinal tract. Spread of Kaposi's to internal organs is the usual cause of complications due to mass effects such as blockage of lymph nodes or blood vessels.
KS in AIDS differs from classic KS in that widespread skin, mucous membrance (especially in the mouth), visceral and lymph node disease occurs.
KS does not need to be treated if it is limited to the skin. Involvement of internal organs requires chemotherapy and/or radiotherapy. The standard treatment for advanced KS is a combination of anti-cancer drugs referred to as "ABV": adriamycin, bleomycin, and vincristine given intravenously, every 2 weeks.
update: there is now some evidence that Kaposi's sarcoma is linked to one of the human herpes viruses - this virus will probably end up being called KS-associated herpes virus or just HHV-8
Kaposi's sarcoma is named for Moritz. K. Kaposi (1837-1902), a Dermatologist from Vienna, Austria who first described this condition.