The unique characteristics of AIDS as a disease transmitted sexually and, less often, by the sharing of needles or by blood transfusions, has created an unprecedented challenge to criminal law. Proof of causation beyond a reasonable doubt is particularly difficult because of the unique biological aspects of AIDS infection and transmission. Sexual contact with more than one partner combined with the lengthy and variable incubation period of the virus makes it difficult to know from whom the virus was contracted. In addition, the fundamental mens rea/actus rea rule of criminal liability will require charges of criminal AIDS transmission to necessitate a showing of at least some degree of intent to transmit the virus. Proof of criminal intent will thus usually involve demonstrating reckless disregard of human safety where the accused is aware of being a carrier but chooses to disregard the risk. Concern that prosecutions will make the epidemic worse by encouraging infected persons to conceal that their infection should not obscure awareness of the need to find, in justifiable cases, criminal liability for criminal acts of AIDS transmission. The difficulties criminal law will face will be in unravelling the unique aspects of this crime; understanding the medical uncertainties of infection, transmission, and detection of carrier status; and ultimately applying criminal rules to acts that result in the transmission of AIDS. 151 footnotes.