Why is AIDS so much more prevalent in Africa than in other parts of the world? The question is both important and controversial. It is important because two-thirds of those infected with HIV, the virus that causes the disease, live on that continent. It is controversial because some of the explanations are behavioural, and are seen by many as racist slurs. Broadly, there are three proposed explanations. One is that because AIDS started in Africa, it has had more time to spread there. The second — the behavioural one — is that both formal and informal polygamy are more common in Africa than elsewhere. The third is that African physiology is unusually susceptible to the disease.

Virtually no part of the world has remained untouched by HIV according to the United Nations Programme on Aids and WHO. More than 60% of all people with HIV live in Sub-Saharan Africa. Despite falls in adult HIV prevalence apparently under way in Kenya, Uganda and Zimbabwe, there is little evidence of declining epidemics in this region as a whole. In fact, prevalence levels remain "exceptionally" high.

It was careful science - starting with a scientific hypothesis, following it up with observations in the field and ending with clinical trials - which proved that male circumcision, common in many parts of Africa, protects against infection by up to 60%. Amid the gloom about microbicides and vaccines, circumcision is the one bright spot in the field of AIDS prevention.

It is hard to believe that the world has been living with the Aids epidemic for a quarter of a century. As 20,000 delegates meet in Mexico City for the 17th International Aids Conference, there is much progress to report, but some setbacks, too. New figures from the United Nations show that, for the second year running, the overall number of people who are HIV-positive has dropped, from around 33.2 million in 2006 to 33 million last year.