At the dawn of the 21st century, the health of the world is at a critical crossroads. Malaria, tuberculosis, and other diseases virtually eliminated from developed countries routinely claim millions of lives in developing nations. More than 40 new or emergent diseases like Ebola fever, SARS, West Nile virus, and avian flu have appeared within the last 30 years. Strains of "superbugs" resistant to our best drugs mystify modern medicine, while HIV/AIDS still lacks a vaccine and continues to ravage the world.