HIV/AIDS is without doubt the worst epidemic to hit humankind since the Black Death. As of 2004 an estimated 40 million people were living with the disease, and about 20 million had died. Despite rapid scientific advances there is still no cure and the drugs are expensive and toxic. In the developing world, especially in parts of Africa, life expectancy has plummeted to below 35 years, causing a serious decline in economic growth, a sharp increase in orphans, and the imminent collapse of health care systems. The news is not all bleak though. There have been unprecedented breakthroughs in understanding diseases and developing drugs. Because the disease is so closely linked to sexual activity...