UNLIKE THE OPTIMISTS of the 1960s, the scientists of today recognize that infectious disease is likely to last as long as humanity—and possibly longer. "We have to come to terms with the fact that the microbial world is in competition with us," says Joshua Lederberg, who won a Nobel Prize in 1958 for his studies of bacterial genes.

Some reasons why epidemics will probably never vanish have to do with the microbes that produce them. For one thing, many disease-causing microorganisms can survive in wild animals as well as in people. Plague bacteria and hantaviruses live in or on rodents, for instance. Influenza lives in wild waterfowl, and cholera bacteria live on tiny sea creatures. Getting rid of all these animals is both impractical and undesirable, and removing their microbe passengers is likely to be equally impossible. Other bacteria and viruses, now unknown, no doubt live in...