The two most common skin cancers, basal cell carcinoma and squamous cell carcinoma, are non-melanomas, which are rarely life-threatening. They progress slowly, seldom spread beyond the skin, are detected easily and usually are curable. Basal cell carcinoma, which accounts for nearly three out of four skin cancers, is the slowest growing. Squamous cell carcinoma is somewhat more aggressive and more inclined to spread. There are also a few rare non-melanomas such as Kaposi's sarcoma, a potentially life-threatening disease characterised by purple growths and associated with a suppressed immune system - it is almost always seen in patients with AIDS.