HIV exists in the form of several different subtypes. In Africa, Asia, and other developing regions, subtypes A, C, D, and E predominate, primarily among heterosexuals, and are responsible for at least twenty million infections. In Europe and North America, B is the primary subtype; it is transmitted more easily among homosexuals and intravenous drug users than among heterosexuals.

In recent years, isolated cases of non-B HIV infection have been detected among heterosexuals in Europe and North America. Harvard AIDS Institute virologist Max Essex is one researcher who warns that because of this emergence of non-B HIV, and because subtypes C and E have been found to readily reproduce in heterosexuals, “we could face a much more significant epidemic among heterosexuals.”

However, other experts disagree with Essex’s warning that non-B subtypes could spark future HIV epidemics among...