In England and Wales, criminal prosecutions for recklessly causing serious bodily harm by transmitting HIV have occurred since 2003. Understanding how people respond to the application of criminal law, will help to determine the likely impact of prosecution. As part of a wider qualitative study on unprotected anal intercourse amongst homosexually active men with diagnosed HIV in England and Wales, 42 respondents were asked about their awareness of criminal prosecutions for the sexual transmission of HIV, and how (if at all) they had adapted their sexual behaviour as a result. Findings demonstrate considerable confusion regarding the law and suggest that misunderstandings could lead people with HIV to wrongly believe that how they act, and what they do or do not say, is legitimated by law. Although criminalisation prompted some respondents to take steps to reduce sexual transmission of HIV, others moderated their behaviour in ways likely to have adverse effects, or reported no change. The aim of the criminal justice system is to carry out justice, not to improve public health. The question addressed in this paper is whether desirable public health outcomes may be outweighed by undesirable ones when the criminal law is applied to a population-level epidemic.