Have you talked to your doctor about sex, lately? Doctors who give pre-travel health advice are generally more inclined to launch into a discussion of the latest research into malaria, or even travellers' diarrhoea, than raise the issue of casual sex. After all, doesn't everybody already know about HIV? The truth is, however, that one important fact about HIV seems to have escaped most people's attention: HIV infection resulting from sex abroad has now overtaken every other tropical and infectious disease hazard to become the single most frequent cause of lethal infection in travellers.

The comparison between HIV and malaria is an interesting one. Malaria's place in the pantheon of travel-related disease, and in the thoughts and fears of departing travellers and their doctors, is secure. There are approximately 2,000 cases of malaria in British travellers every year, causing between ten and 15 deaths. We agonise over each fatal case that occurs in the near-certain knowledge that it could have been prevented, and we scrutinise every detail for lessons that can be learned for the future. Yet during 1999 there were 969 confirmed cases of HIV infection resulting from heterosexual sex abroad. These cases include migrants to the UK, not just travellers; but in view of the long delay before infection becomes apparent, they represent the tip of a much larger iceberg: the true rate of infection is probably twice this number. The HIV infection rate in travellers is currently 100 times higher than the fatality rate from malaria, and it is rising.

These figures mean that no fewer than 85 per cent of British HIV cases in heterosexuals are acquired abroad. So far, most of these overseas infections have originated in Africa, but other regions are catching up - especially Asia and the countries of the former USSR.

In developing countries worldwide, the rates of HIV infection are mounting, and the pattern of infection is also changing. HIV is increasingly a disease of the young: some 60 per cent of new infections are in people aged between 15 and 24. Young women are particularly at risk: in Uganda, for example, the rate of infection among women in the 13 to 19 age group is 20 times higher than it is in men, and a recent survey of pregnant teenage women in Zimbabwe found 30 per cent of them to be HIV positive. Among people of both sexes in the commercial sex industry, infection rates in many parts of the developing world approach 100 per cent.

Other sexually transmitted diseases, from gonorrhoea and syphilis to more exotic and unfamiliar diseases such as chancroid, may seem to pale into insignificance beside the risks of HIV, but they have not disappeared from the scene: STDs are now nearly as common as malaria, with more than 250 million new cases worldwide every year: each year, one in 20 adolescents worldwide contracts a sexually transmitted disease.

How have travellers responded to these growing risks? In a survey of 782 returning travellers at the Hospital for Tropical Diseases, in London, 18.6 per cent reported having sex with at least one new partner while away, almost half of them with more than one partner. Another, smaller survey, found that 12 out of 17 people who admitted to having sex with a new partner while abroad had carried condoms, but had failed to use them on account of getting very drunk. A recent Swiss study found that four per cent of Swiss visitors to Kenya had sex with local people, often without using condoms, while a survey of Swedish travellers found that 28 per cent had sex with a new partner while travelling in Europe. These figures appear to reflect remarkable levels of restraint by comparison with a British survey of visitors to Torquay, where, out of 1,000 people aged 1629, 600 said that they had had sex with a new partner without a condom during their visit. (Another finding from the same study: holiday-makers who were engaged to be married but had left their partners at home were more likely to report a sexual encounter than other visitors.)

Sex with fellow travellers rather than local people is not necessarily an entirely safer option since surveys have shown that many people are prepared to lie in order to have sex. One survey of young, sexually active Californians showed that 47 per cent of the men and 60 per cent of the women claimed that they had been lied to for the purposes of sex; 34 per cent of the men and ten per cent of the women admitted that they themselves would also be prepared to lie. Twenty per cent of the men said that they would lie about having a negative HIV- antibody test, and nearly half of both men and women said that they would understate their number of previous partners. Of those who had been sexually involved with more than one person at a time, more than half said their partners did not know.

In Thailand, where the sex tourist once reigned supreme, public attitudes are only now beginning to change. Largely through the efforts of former government minister Mechay Viravaidya (widely known in his country as Mr Condom), an energetic programme of public education and condom distribution is under way. In the Philippines, a British travel agent received a 16-year prison sentence for organising paedophile sex tours into the country as part of a new drive by President Fidel Ramos to curb sex tourism. Such initiatives are still relatively rare in the developing world, and will have to be carefully planned, vigorous and sustained before they will have any real impact upon the health of local people.

When Edwina Currie advised travellers to avoid temptation by taking along their partner, she at least managed to hit the headlines. Current British efforts to educate departing travellers consist of distributing soft-sell leaflets to GP waiting rooms and an occasional poster at Heathrow. Until we can find more effective ways to increase public awareness of the problem, and to persuade travellers to avoid or reduce their exposure, HIV infection will remain the most formidable disease hazard of modern international travel.