In Nyanza, a province in Western Kenya, a green valley has become the testing ground for a new theory on the AIDS epidemic. On one side of this valley, 60 percent of the population will likely fall victim to the disease. On the other side, the infection rate is three times less. Sexual habits are similar on both sides, condom and drug use are similar on both sides, diet and medical histories are similar on both sides. Residents and international experts were baffled. It would take years of anthropological, sociological and biological study for an answer to emerge. Circumcision seemed to help men avoid contracting HIV, but how?