PIP: The accumulated data strongly support the conclusion that transmission of HIV occurs through blood, sexual activity, and perinatal events. Nevertheless, the fear of transmission by other routes may continue to increase with the anticipated increase in the number of cases of AIDS over the next few years. An unrealistic requirement for absolute certainty about the lack of transmission by other routes persists, despite the knowledge that it is not scientifically possible to prove that an event cannot occur. It remains difficult to believe that a virus that is spreading rapidly and may cause a cruel, frightening, and fatal disease is not highly contagious and easily transmitted. The available data indicate that HIV transmission is not highly efficient in a single or a few exposures, unless one receives a very large inoculum. The widespread dissemination of HIV is more likely the result of multiple, repeated exposures over time by routes of transmission that are strongly related to personal and cultural patterns of behavior--particularly, sexual activity and the use of drugs. Isolated transmission events should be placed in full perspective by examining the results of population-based studies that provide rates of risk. It is from these studies that rational approaches and public policy should be developed.