There are only three known routes of transmission of the human immunodeficiency virus (HIV): (1) exposure to blood via transfusion, sharing of contaminated needles by drug abusers, occupational needle stick or blood spill, unsterile needle injections; (2) sexual transmission; (3) perinatal exposure. There is no evidence for transmission by close interpersonal nonsexual contact or insect vectors. Health care workers are at risk for HIV transmission through accidental parenteral inoculation or extensive exposure to blood on nonintact skin or mucous membranes. Fortunately, population studies indicate that this risk is low and may be further reduced by adherence to simple infection control guidelines. The accumulated information on low rates of occupational transmission of HIV makes unwarranted the treatment of patients with acquired immunodeficiency syndrome (AIDS) or HIV infection as if they were highly contagious in the health care setting.