Given that some bacteria and viruses treat us mainly as meals, we Gcan think of them as enemies. Unfortunately, they are not just simple pests put here to bedevil us but sophisticated opponents. We have evolved defenses to counter their threats. They have evolved ways to overcome our defenses or even to use them to their own ben- efit. This endlessly escalating arms race explains why we cannot erad- icate all infections and also explains some autoimmune diseases.
This distinction between defenses and defects is not merely of aca- demic interest. For someone who is sick it can be crucial. Correcting a defect is almost always a good thing. If you can do something to make the clanking in the transmission stop or the pneumonia patient’s skin turn warm pink, it is almost always beneficial. But eliminating a defense by blocking it can be catastrophic. Cut the wire to the light that indicates a low fuel supply, and you are more likely to run out of gas.