RESUMEN
There has long been reason to anticipate a major heterosexual epidemic of acquired immunodeficiency syndrome (AIDS) in Papua New Guinea (PNG) and probably in the rest of Melanesia. From the social and behavioural perspectives, Melanesia is strikingly similar to other areas of the world with serious epidemics of AIDS. High levels of other sexually transmitted infections indicate behaviour patterns that would also facilitate transmission of human immunodeficiency virus (HIV) and presence of cofactors for HIV infection. Low levels of male circumcision parallel the situation in other epidemic areas. Near-parity by sex in cases reported so far in PNG is evidence that primary infection is largely heterosexual. The late start of a major epidemic in PNG can probably be attributed to: (a) the relatively small aggregation of people in urban centres (even Port Moresby has only one-quarter of a million people); (b) a highway system that does not network across the whole country; (c) limited size of the organized commercial sex sector; and (d) possibly low level of chancroid to act as a cofactor. The situation is now changing. Over the last seven years, HIV infection, probably the highest in Port Moresby and mostly measured there, has been rising by about 60% per annum. This rise is genuine and, if sustained, would infect 10% of the adult population of PNG in little more than 12 years. Some countries of sub-Saharan Africa have witnessed such exponential rises.