After a lapse of some decades, germs and disease have again been very much on our minds, largely because of the dreadful effect of AIDS throughout the world. We also have had a reawakened consciousness that globally prevalent diseases like tuberculosis and malaria remain historical scourges. Now the daily news tells us of new outbreaks such as severe acute pulmonary syndrome, or SARS, spreading from China, with an outcome that cannot be confidently predicted at this time.
Throughout history, infectious disease has regulated lives. Only in the 20th century, thanks to simple hygienic measures like washing our hands regularly and separating drinking water from sewage runoff, have we taken a larger role in trying to control how microbes affect human life.
A child born in the United States in 1900 had an average life expectancy of 47 years. By the end of that century, mainly because of our...