In 1967 the surgeon general of the United States delivered a speech to Congress: "It is time to close the book on infectious diseases," he said. "The war against pestilence is over."

In 1998 Surgeon General David Satcher had a different message. The Miami Herald reported his speech with this headline: "Infectious Diseases a Rising Peril; Death Rates in U.S. Up 58% Since 1980."

The middle of the last century was a time of great confidence in science and medicine. With the introduction of antibiotics in the 1940s, and a lengthening list of vaccines that prevented the most frightening diseases, Americans felt that it was only a matter of time before diseases caused by microorganisms (i.e., infectious diseases) would be completely manageable. The nation’s attention turned to the so-called chronic diseases, such as heart disease, cancer, and stroke.

So what happened to change the optimism of the 1960s to the warning expressed in the speech from 1998? Dr. Satcher explained it this way: "Organisms changed and people changed." First, we are becoming more susceptible to infectious disease precisely because of advances in medicine. People are living longer. Sicker people are staying alive much longer than in the past. Older and sicker people have heightened susceptibility to what we might call garden-variety microbes. Second, the population has become more mobile. Travelers can crisscross the globe in a matter of hours, taking their microbes with them and introducing them into new "naive" populations. Third, there are growing numbers of microbes that truly are new (or at least, new to us). The conditions they cause are called emerging diseases. Changes in agricultural practices and encroachment of humans on wild habitats are just two probable causes of emerging diseases. Fourth, microorganisms have demonstrated their formidable capacity to respond and adapt to our attempts to control them, most spectacularly by becoming resistant to the effects of our miracle drugs.

And there's one more thing: Evidence is mounting that many conditions formerly thought to be caused by genetics or life-style, such as heart disease and cancer, can often be at least partially caused by microorganisms.

Microbes never stop surprising us—in their ability to harm but also to help us. The best way to keep up is to learn as much as you can about them. This book is a good place to start.