Assumptions can be dangerous, particularly when it comes to disease. And so it’s important to note that at the AIDS 2008 conference experts have challenged numerous preconceptions about HIV transmission.

One example of this involves risks relating to heterosexual sex. It’s no secret that having unsafe sex is similar to engaging in a round of Russian roulette. Scientists have often cited the estimate that one encounter of unprotected heterosexual intercourse with an infected individual carries a 1 in 500 to 1 in 1,000 chance of HIV transmission. But results presented here in Mexico City suggest that in certain circumstances the risk is much, much higher than that.

Myron Cohen of the University of North Carolina, Chapel Hill spoke here yesterday about data from a recent systematic review he helped conduct that suggests the story is far more complex. One study he and his colleagues looked at reported a rate of HIV transmission of once out of every 3.1 acts of heterosexual anal intercourse. They also found the rate was higher in cases where a partner had genital ulcers, for example, which facilitate the spread of virus.

So while the 1 in 1,000 figure might apply to some heterosexual encounters, it certainly does not apply to all of them.

In recent weeks we’ve heard announcements about increases in funding towards the treatment HIV/AIDS. At the G8 summit in Hokkaido, Japan last month, world leaders set the target of spending $60 billion over the next five years towards tackling a handful of diseases, including HIV/AIDS. And just last week US President George W. Bush reauthorized an augmented version of the President’s Emergency Plan for AIDS Relief (PEPFAR), which will is slated to supply $48 billion through 2013 to help fight AIDS, tuberculosis and malaria in regions such as sub-Saharan Africa.

But with increased spending comes increased scrutiny of returns on investment—and that’s very much a topic of discussion here at the AIDS 2008 meeting.

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