If such forms of the virus exist it could explain why some parts of the world, such as Africa, have a heterosexual Aids epidemic, while elsewhere the virus is prevalent among drug users but is failing to reach the heterosexual population as quickly as predicted.
Two distinct types of the HIV-1 virus have been identified in Thailand in two groups who contracted it in a different way. More than 80 per cent of those infected heterosexually had a Type A virus (mainly in northern Thailand) while 76 per cent of those infected through injecting drugs had Type B. Type A is related to HIV found in Central Eastern Africa while Type B is related to that occurring in northern Europe and the US.
Dr Chin-Yih Ou, of the Centres for Disease Control, Atlanta, said it was 'very, very unusual' to have such segregation of viral types. 'We do not fully understand the reason for this segregation. It could be that there is little high risk contact between drug users and prostitutes and the HIV epidemic is occurring quite separately in these groups. Or more speculatively, there may be biological differences in the degree of transmission of the two sub- types.' In other words one type may be adapted to transmission through blood, while the other sub-type is more likely to be transmitted through heterosexual intercourse.
Thailand was the first country in Asia to be affected by the HIV pandemic, beginning in 1988.