Prevention of HIV infection requires a thorough understanding of the modes of viral transmission, the populations at risk, and the established guidelines to avoid high-risk exposures. HIV has been identified in virtually every body fluid and tissue, including blood, semen, vaginal secretions, saliva, tears, breast milk, cerebrospinal fluid, amniotic fluid, urine, and fluid obtained from bronchoalveolar lavage. In most instances, the virus resides in lymphocytes present within body fluids; therefore, any fluid that contains lymphocytes could be implicated theoretically in the spread of the virus. Nonetheless, no cases of HIV transmission have been documented through any body fluids except blood and fluids grossly contaminated with blood, semen, vaginal secretions, and, rarely, breast milk. HIV has been transmitted through transplanted organs, including kidney, liver, heart, pancreas, and bone.