THE AIDS epidemic could be set to surge in richer societies that have so far escaped the worst strains of the disease. That is one conclusion of research that shows Asia and Africa are infested with a much more infectious strain of the disease than the West. The initial horror of the epidemic has begun to fade in many developed countries as the virus has failed to make its mark in society outside the intravenous drug user and homosexual populations. Until now ignorance and poor living standards were thought to explain the AIDS explosion in Asia and Africa. Untreated venereal sores, multiple sex partners and the infrequent use of condoms, all of which increase the chances of infection, were blamed. But Professor Max Essex, chairman of the Harvard AIDS Institute, has discovered there are effectively two epidemics: one which is transmitted primarily through blood cells, and one that is passed through mucosal cells. 'This explains why the risks of heterosexual transmission appear so much higher in Africa and Asia than in the West,' he said. There may be a roughly one-in-1,000 chance of infection during heterosexual sex with the HIV-1 B type virus that predominates in the West; but in Asia (A type), India (C type) and Thailand (E type) the infection risk rises dramatically to between one-in-10 and one-in-20. German Health Minister Horst Seehofer was recently forced to deny accusations of scaremongering after his officials warned highly infectious forms of the disease had been introduced into the country by sex tourists returning from Thailand. AIDS was unknown in Thailand 10 years ago, but the World Health Organisation (WHO) estimates 800,000 Thais are now infected with the virus. Ninety per cent of them are heterosexuals. By their own government's estimates, Germans account for one in 10 of the 900,000 or so 'sex tourists' who visit Thailand every year. There has been a sharp decline in the infection rate among German homosexuals partly due to government funding of self-help groups and AIDS education. But heterosexuals have been slow to take up the safe sex message, mistakenly believing their chances of infection are too low to merit precautions. Professor Essex warned: 'If Asian or African strains infect a large number of people in the West, then it is conceivable we might see it spread rapidly there.' The B type strain requires some exchange of blood for infection, but the other types appear able to pass easily through mucosal cells in the vagina and penis. Luc Montagnier, the man credited with isolating the virus, of the Pasteur Institute in France, said Professor Essex's work was probably correct. But he believed some societal differences remained: 'People living in the North [the richer countries], with better hygiene, have poorly stimulated immune systems, so there are relatively few immune cells for the virus to infect. In the South - with microbes and poor hygiene - there are many more immune cells around to be infected.' This might explain, he said, why people living in poor ghettos are most at risk in the West. Even so, he warned if heterosexuals in richer countries did not remain on the alert, there could be another surge in the epidemic.