This week the pope is in London. You will have your own views on the discrimination against women, the homophobia, and the international criminal conspiracy to cover up for mass child rape. My special interest is his role in the 2 million people who die of Aids each year.

In May 2005, shortly after taking office, the pope made his first pronouncement on Aids, and came out against condoms. He was addressing bishops from South Africa, where somebody dies of Aids every two minutes; Botswana, where 23.9% of adults between 15 and 49 are HIV positive; Swaziland, where 26.1% of adults have HIV; Namibia (a trifling 15%); and Lesotho, 23%.

This is continuing. In March 2009, on his flight to Cameroon (where 540,000 people have HIV), Pope Benedict XVI explained that Aids is a tragedy "that cannot be overcome through the distribution of condoms, which even aggravates the problems". In May 2009, the Congolese bishops conference made a happy announcement: "In all truth, the pope's message which we received with joy has confirmed us in our fight against HIV/Aids. We say no to condoms!"

His stance has been supported, in the past year alone, by Cardinal George Pell of Sydney and Cardinal Cormac Murphy O'Connor, the Archbishop of Westminster. "It is quite ridiculous to go on about Aids in Africa and condoms, and the Catholic Church," says O'Connor.

"I talk to priests who say, 'My diocese is flooded with condoms and there is more Aids because of them.'"

Some have been more imaginative in their quest to spread the message against condoms. In 2007, Archbishop Francisco Chimoio of Mozambique announced that European condom manufacturers are deliberately infecting condoms with HIV to spread Aids in Africa. Out of every 8 people in Mozambique, one has HIV.

It was Cardinal Alfonso López Trujillo of Colombia who most famously claimed that the HIV virus can pass through tiny holes in the rubber of condoms. Again, he was not alone. "The condom is a cork," said Bishop Demetrio Fernandez of Spain, "and not always effective."

In 2005 Bishop Elio Sgreccia, president of the Pontifical Academy for Life, explained that scientific research has never proven that condoms "immunise against infection".

He's right, they don't. They stop the virus which kills you from being transmitted during sex.

How effective are they? It's wise not to overstate your case. The current systematic review of the literature on this question published by Cochrane found 14 observational studies (because it's unethical to do a randomised trial where you actively stop people using condoms, since you know that they work but just want to find out how well).

These studies generally looked at HIV transmission in stable couples where one partner had HIV.

Many of them looked at transfusion patients and haemophiliacs. Overall, rates of HIV infection were 80% lower in the partners who reported always using a condom, compared to those who said they never did. 80% is pretty good.

There is no single perfect solution to the problem of Aids: if things were that easy, it wouldn't be killing 2 million people every year.

ABC is a widely used prevention acronym in Africa: abstain, be [faithful], [use a] condom. Picking out one effective measure and actively campaigning against it is plainly destructive, just as telling people to abstain doesn't make everyone abstain, and telling people to use condoms won't make everyone use them. But Ratzinger has proclaimed: "The most effective presence on the front in the battle against HIV/Aids is the Catholic church and her institutions."

This is ludicrous. You, the Catholic church, is the only major influential international political organisation that actively tells people not to do something that works – on a huge scale. Your own figures show that your numbers are growing in Africa, even faster than the population does.

I'm happy for you to suggest abstention. But sabotaging an effective intervention which prevents a disease that kills 2 million people a year makes you a serious global public health problem.