The enthusiastic quest to vanquish disease is a common, if unconscious, thread in much of what is written about medical biotechnology. Given the extraordinary progress made in diagnosing and treating many previously untreatable conditions, it's understandable that people expect the trend to continue ever onward and upward. And yet, during the same period of this medical revolution, we've also seen the resurrection of old diseases once thought conquered, such as tuberculosis and polio. There has been an increase in certain cancers, and the advent of some new and even more virulent diseases, such as AIDS and the Ebola virus. Why is that?

To think of disease as defeatable is to think of it as a fixed set of more-or-less inert objects, like bottles on a wall, which researchers with shotguns can pick off one at a time. Ten down, 990 to go. In the more dynamic world of living things, however, the bits of broken glass at the wall keep re-assembling to form new bottles, forever ready to pop back into empty spaces.

Almost as soon as penicillin became widely used, for example, antibiotic-resistant germs began to appear. These resistant strains are simply microbes that our own strategy of defense helps promote. They are mutant survivors of our chemical blitz, passing on their techniques of resistance to subsequent generations to produce new microbes that have escaped antibiotic control. Since microbes have lifetimes measured in minutes or hours rather than years or decades, resistance can take hold and spread very rapidly.