The G8 summit in Germany is due to focus particularly on Aids. Bill Gateswrites that education and contraceptives are the way to save lives
On a recent visit to a clinic in Africa, my wife Melinda and I were shown a heartbreaking photograph of a patient named Jean - a very thin, frail man suffering from Aids.
As we were staring at the photo, a smiling man walked into the clinic to greet us. It took us a minute to realise that it was Jean - now alive and healthy, thanks to powerful new Aids drugs.
Jean is not alone. According to a new report, the number of people on Aids treatment in developing countries doubled in the past year. This is largely due to the generosity of donor countries, including Ireland, which has committed €80 million to the Global Fund to Fight Aids, TB, and Malaria.
If everyone personally could meet an African whose life has been saved by Aids treatment, I think we would all be deeply proud of what has been accomplished, and push government leaders to build on this success.
But treatment is only part of the equation. To secure Africa's future, we must also do a much better job of preventing the spread of the disease - especially among women.
The harsh mathematics of Aids proves the importance of prevention. For every person who gained access to treatment last year, six more were infected. Without slowing the number of infections, there's simply no way to keep up with the surging demand for treatment.
Sceptics say it is not possible to change the behaviour that puts people at risk of HIV. I defy them to tell that to the sex workers we have met on our trips to Africa, who insist their customers use a condom every time.
Years of research have proven that Aids education, condoms, clean needles, and HIV testing can be highly effective in preventing the spread of HIV.
A report to be issued next month by the Global HIV Prevention Working Group estimates that with a major expansion of prevention, we could cut the number of infections by two-thirds within a decade.
But it's shocking how few people - even those at extremely high risk - have access to these lifesaving prevention tools. Worldwide, most schoolchildren don't receive any Aids education. Just one in 10 adults in Africa has been tested for the virus.
And only one in 10 pregnant women has access to inexpensive medications that can protect newborn babies from infection.
A top priority must be to address the prevention needs of women and girls, especially in Africa. Biologically, women are twice as likely as men to contract HIV. And many women - including those who are married - have little power to ensure their partners are faithful or use condoms.
A woman shouldn't need her partner's permission to save her own life. I am hopeful that science and technology will put the power to prevent HIV in the hands of women.
Promising research is under way on microbicides - gels that a woman could use to prevent HIV transmission. It is also possible that drugs used to treat HIV could be used to prevent infection. And of course, the development of an Aids vaccine would be the ultimate breakthrough.
In addition to developing new prevention methods, we must also address more fundamental issues, such as sexual violence and social and economic inequities, that put women and girls at risk of HIV. It is no coincidence that girls with better access to education are also less likely to become infected.
We also cannot forget the importance of HIV testing - which is essential to the success of both treatment and prevention. The World Health Organisation should be commended for issuing new guidelines last week that call for greatly expanded access to testing.
Fortunately, fighting Aids in Africa is a priority at this week's G8 summit. Donor countries should take this opportunity to pledge new resources for effective prevention and treatment programmes, and help fast-track research on new prevention methods.
Controlling, and one day eliminating, Aids is the number-one priority for the Gates Foundation. We won't stop until there's an end to the disease, and we hope that the world's most powerful nations won't either.
Bill Gates is chairman of Microsoft and co-chair of the Bill & Melinda Gates Foundation