This disease affects the vasculature. It consists of multiple lesions with a purplish-brown color which look like plaques, maculae or papulae (thickenings of the skin). These lesions can coalesce to form large tumors on the skin.

Four forms of Kaposi's have been identified:
i) Classic or Mediterranean form of Kaposi's affects elderly men of Mediterranean, Eastern European or Jewish descent and is rare in the United States.  It causes little pain i.e. it is a benign, indolent tumor. Tumors are often limited to the extremities, usually the lower extremities. 
ii) Post-transplant or iatrogenic Kaposi's develops after renal transplantation in some patients on cephalosporin. This occurs after some months of immunosuppressive therapy. This type of Kaposi's can regress when the immunosuppressive therapy ceases.
iii) Endemic or African Kaposi's is found in southern Africa. It is much more aggressive and affects internal organs as well as the skin. This form of the sarcoma causes high morbidity. In African children, there is a form of endemic Kaposi's that aggressively affects the lymph nodes and is rapidly fatal.
iv) AIDS-associated Kaposi's occurs after HIV-1 infection and is particularly found in HIV-infected men who have sex with men. Like endemic Kaposi's, it is very aggressive, disseminates to internal organs and is fatal. The disease is rare in HIV-2-infected patients. It is thought that the Tat protein of HIV-1 is responsible for the aggressive properties of AIDS-associated Kaposi's. 

Kaposi's sarcoma can also occur in young gay men who are not HIV-infected

It should be noted that, although there are these four forms of Kaposi's sarcoma with very different outcomes, they all share identical histology that involves growth of new blood vessels (neoangiogenesis) and edema. The tumor becomes infiltrated with monocytes and elongated spindle cells (of disputed origin) appear.

All types of Kaposi's seem to involve a problem with the immune system, that is some form of immune-suppression. In all cases there is activation of CD8 T cells and elevated expression of certain cytokines. The inflammatory aspect of Kaposi's may be triggered by Human Herpes Virus-8 and promoted by Human Immunodeficiency Virus.