Worldwide an estimated 33 million people are living with HIV. The popular explanations include extreme poverty, lack of HIV education, and insufficient access to condoms and health care. But conventional wisdom and certain assumptions can't entirely explain the AIDS pandemic. For a deeper look at what the biological and behavioral factors are that put so many at risk for HIV/AIDS, The Takeaway turns to Daniel Halperin, a leading expert on HIV prevention. Daniel Halperin is a lecturer in Global Health and Senior research scientist at Harvard University School of Public Health.