Nearly half of Swiss people recently infected with HIV contracted the virus from their partner, according to a study released ahead of World Aids Day on December 1.
Two decades after Aids was identified, the vast majority of HIV infections occur during unprotected sex, often between couples in stable relationships.
The study, commissioned by the Federal Health Office, sought to understand how the Aids epidemic is spreading now.
People diagnosed with HIV within five months of becoming infected were asked when and where they were likely to have contracted the virus. Almost half said they were infected by their (former or present) partner.
Nearly all heterosexuals questioned said they did not contract HIV through a casual sexual encounter.
Led by experts from St Gallen Cantonal Hospital, the study showed that approximately 86 per cent of respondents were infected after unprotected sex, while another seven per cent caught HIV from sharing syringes.
Many had a so-called efficient protection strategy and should have relied on safe sex practices. But they were infected because they were under the influence of drugs or alcohol, failed to insist on condom use, were caught up in the moment, in too much of a hurry, or simply trusted their partner.
"If the steady partner was the source of infection, then that trust was most certainly misplaced," said Pietro Vernazza, head of the project. "You shouldn't necessarily trust your partner, rather you should trust an HIV test."
Others had an unreliable strategy, such as relying on physical appearances, while some had no strategy whatsoever, believing they were not at risk from HIV.
"The study shows we can get a lot more information about HIV infection by asking individuals just a few more questions," added Vernazza. "It will improve our understanding of the epidemiology and help improve prevention."
According to Vernazza, the study did not aim to evaluate current prevention measures, but provide future tools. He did admit though that the message about testing soon after a potential infection was not necessarily getting through.
"If we realise that two thirds of newly diagnosed HIV cases are infections that took place over five months ago, then we have missed a window of opportunity," he told swissinfo. "HIV is far more infectious in the first months."
The researchers say the use of the questionnaires they have developed should become a routine matter.
They also recommend pursuing preventive measures among the long-term resident population, while targeting risk groups such as gay and bisexual men, drug users and migrants from certain regions with voluntary testing and counselling.
The Federal Health Office will be promoting this message next year, but it is counting on physicians to overcome their inhibitions to spread the message.
"Especially with the heterosexual population, doctors need to think more about HIV testing," said Vernazza. "They should act in particular when a patient shows any sign of possible infection."
swissinfo, Scott Capper
Total cases of HIV infection in Switzerland between 1985 and 2005: 28,612.
Diagnosed cases in 2005: 708.
Over half of new infections came from heterosexual contacts, one third from homosexual relations.
Aids cases between 1983 and 2005: 8,251.
Aids deaths during the same period: 5,622.
The survey, which ran from July 1, 2005 until June 30 this year, was based on responses from patients who tested HIV positive at 11 authorised laboratories.
Participation was voluntary, with 140 people responding to an intial questionnaire out of 361 who were contacted. Another 21 agreed to further questioning.
The cantonal hospital of St Gallen, the Swiss Aids Federation and the Association of Cantonal Health Officers took part in the government-funded programme, which cost SFr300,000 ($250,000).