Most people, when they thought of bacteria, thought of diseases. Yet the fact was that only 3 percent of them produced human disease; the rest were either harmless or beneficial. In the human gut, for instance, there were a variety of bacteria that were helpful to the digestive process. Man needed them, and relied upon them.

In fact, man lived in a sea of bacteria. They were everywhere—on his skin, in his ears and mouth, down his lungs, in his stomach. Everything he owned, anything he touched, every breath he breathed, was drenched in bacteria. Bacteria were ubiquitous. Most of the time you weren't aware of it.

And there was a reason. Both man and bacteria had gotten used to each other, had developed a kind of mutual immunity. Each adapted to the other.

And this, in turn, for a very good reason. It was a principle of biology that evolution was directed toward increased reproductive potential. A man easily killed by bacteria was poorly adapted; he didn't live long enough to reproduce.

A bacteria that killed its host was also poorly adapted. Because any parasite that kills its host is a failure. It must die when the host dies. The successful parasites were those that could live off the host without killing him.

And the most successful hosts were those that could tolerate the parasite, or even turn it to advantage, to make it work for the host.

"The best adapted bacteria," Burton used to say, "are the ones that cause minor diseases, or none at all. You may carry the same single cell of Strep. viridians on your body for sixty or seventy years. During that time, you are growing and reproducing happily; so is the Strep. You can carry Staph. aureus around, and pay only the price of some acne and pimples. You can carry tuberculosis for many decades; you can carry syphilis for a lifetime. These last are not minor diseases, but they are much less severe than they once were, because both man and organism have adapted."

It was known, for instance, that syphilis had been a virulent disease four hundred years before, producing huge festering sores all over the body, often killing in weeks. But over the centuries, man and the spirochete had learned to tolerate each other.

Such considerations were not so abstract and academic as they seemed at first. In the early planning of Wildfire, Stone had observed that 40 per cent of all human disease was caused by microorganisms. Burton had countered by noting that only 3 per cent of all microorganisms caused disease. Obviously, while much human misery was attributable to bacteria, the chances of any particular bacteria being dangerous to man were very small. This was because the process of adaptation—of fitting man to bacteria—was complex.

"Most bacteria," Burton observed, "simply can't live within a man long enough to harm him. Conditions are, one way or another, unfavorable. The body is too hot or too cold, too acid or too alkaline, there is too much oxygen or not enough. Man's body is as hostile as Antarctica to most bacteria."