As a badly lapsed Catholic, I don't often agree with the Pope. So I was surprised to find myself nodding when, on the eve of his first visit to Africa as pontiff, he pronounced that HIV/Aids was “a tragedy that cannot be overcome by money alone, that cannot be overcome through the distribution of condoms”.

So far so good. Neither cash nor simply handing out condoms will stop the spread of a fatal virus that has infected more than 70 million people worldwide, two thirds in Africa.

What came next was the shocker. According to the Pope, condoms “can even increase the problem”. Talking to journalists en route to Cameroon, he did not back this up.

I've worked with HIV prevention data for more than a decade, and I have found nothing to support this except a claim by William Bennett, a former Secretary of Education in the Reagan Administration, who once pointed out that condom use was higher in communities with higher HIV prevalence - clear evidence that condoms aggravate the epidemic. Similarly, more people use treated bed nets in Lagos than in London, and Nigeria has far more malaria than the UK - clear evidence that bed nets spread malaria.

The Pope is not daft enough to make this basic scientific error. He is even right that giving out condoms does not solve the HIV problem. Using them in sex between infected and uninfected people does, however, make an important contribution to reducing its spread. The Pope may not like that, but disputing its truth makes him seem either ill educated or ill intentioned.

What is his alternative? He is trying to sell us “spiritual and human awakening” and “friendship for those who suffer”. Here, for me, is the great mystery of the Christian approach to HIV. In the Pope's world view, one shared by many evangelicals, we must respond to Aids by showing compassion for its “victims”.

Why can't we also show compassion to uninfected people by helping them to stay that way, using every effective tool at our disposal? That includes abstinence, which works for many Catholic priests and some teenagers. And cutting down the number of people you have sex with. And condoms.

I would like to remind His Holiness that Africa is the only region in which his Catholic empire is growing, by more than 40 per cent in the past two decades. Neither obliging everyone to abstain from sex nor exposing them to death by discouraging condom use in high-risk partnerships is good for his business.

Elizabeth Pisani is an epidemiologist and author of The Wisdom of Whores: Bureaucrats, Brothels and the Business of Aids (Granta)

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