Needle-sharing is another way of exchanging bodily fluids, LazySmoker, albeit in smaller amounts than through sexual contact. A needle goes into an HIV-positive person's arm, picking up some of the virus from that first person's blood or plasma or whatever it comes in contact with beneath the skin. Then the virus lives long enough to be transferable when that needle is put into another person's arm, innoculating person #2 with the virus. Everyone's right that the virus doesn't live long outside a human host, but often needle-sharing is done in a fairly fast succession, with one person using the needle, then another, and then another. So in those cases where HIV gets transmitted, the needles haven't been sitting idle for a period of hours.