A sarcoma is a malignant tumor (i.e. cancer) of connective tissue – material that surrounds body structures and holds them together. Sarcomas are composed of densely packed cells, diffusely embedded in a homogeneous ground substance. Their degree of malignancy varies greatly. They spread by local infiltration and by blood stream invasion. Typical sarcomas include osteosarcoma (arising in bone), chondrosarcoma (arising in cartilage, Kaposi's sarcoma (which mainly affects the skin and is common in people who have AIDS), and fibrosarcoma. Sarcomas are much less common forms of malignant tumors than are carcinomas.