Prisons are recognised worldwide as important sites for transmission of blood-borne viruses (BBVs). There are two reasons why transmission risks in prison are higher than in the community. First, in most western countries, many prison entrants have histories of injecting drug use, and thus already have high prevalences of BBVs. Second, the lack or under-supply of preventive measures (such as clean needle and syringes or condoms) in most prisons, combined with extreme social conditions, creates extra opportunities for BBV transmission. HIV prevalence in prisoners in more developed countries ranges from 0.2% in Australia to over 10% in some European nations. There are case reports of HIV being transmitted by sharing injecting equipment and sexual activity. Tattooing has been reported as a risk factor for the transmission of BBVs in prison. Access to condoms and needle and syringe programmes in prisons is extremely limited, despite success when they have been introduced. The vast majority of prison inmates are incarcerated for only a few months before returning to the community, thus they are, over the long term, more appropriately regarded as ‘citizens’ than ‘prisoners’. Public health policy must involve all sections of the community, including prison inmates, if we are to reduce transmission of HIV and other BBVs.