At the beginning of the 20th century, people in the United States had an average life span of about 50 years. By the time the century neared its close, average life span had risen to 76 years. Other developed countries experienced similar increases. Much of the credit for these longer life spans-and for the good health that accompanies them-is due to the conquering of diseases, thanks to vaccines, antibiotics, sophisticated surgical tools, and other medical miracles. The challenges ahead include bringing the benefits of this medical knowledge to all peoples of the world, and expanding on current knowledge in order to understand, treat, and prevent the diseases that still confront us.