Sally, that is exactly why the large scale African studies were done - to test the hypothesis that circumcision could reduce HIV infection, and if so, to measure how many new infection cases could be avoided through circumcision. The answer was, iirc, between 60 and 75%, depending on the study.
Misspixie, you have mentioned Nigeria several times. Did you know that in African countries where circumcision is highly prevalent (such as Nigeria) the HIV+ rate is up to 12 times lower than in countries where circumcision is not widespread?
In Botswana and Zimbabwe, where there is no circumcision culture, 1 in 4 of the total population is HIV+. In Nigeria, where over 80% men are circumcised, 1 in 25 is HIV+.
If circumcision does not play a causal role in reducing HIV transmission in Nigeria, what, in your view, explains this difference?
This table sets out the rates, country by country. Although it is very sobering, it still does not give a full picture of the severity of the HIV epidemic - such as the fact that nearly one in three women of childbearing age in Botswana is HIV+, or the fact that in the 6 years up to 2005, 17% of health care professionals in Botswana were lost to HIV.