Acquired Immune Deficiency Syndrome (AIDS), disease that renders the body's immune system unable to resist invasion by several microorganisms that cause serious infections. It is usually characterized by severe weight loss and fatigue, and frequently by neurological complications due to damage of cells of the brain. There is also a high incidence of certain cancers, especially Kaposi's sarcoma, which shows up as purple lesions on the skin, and tumors known as B-cell lymphomas. AIDS is transmitted by blood, through intimate sexual contact, from infected mothers to their babies in the uterus, and perhaps through infected mother's milk. Before a reliable test for screening blood was developed, a major route of transmission was through receiving transfusions of contaminated blood. A major means of transmission and spread of the virus is through the use of blood-contaminated needles by intravenous drug abusers. Casual contact in general is not a risk factor for infection, and blood donors are definitely not at risk of catching the disease. The virus usually remains dormant for some time in infected T cells, and it may take up to 10 years for symptoms to develop.