Safety in diversity
Motley's the only wear.
Jaques, in Shakespeare's As You Like It.
Consider the AIDS viruses once again, and their fierce struggle against the immune system of each person they infect. Much of that viral evolution appears to be a response to the fresh challenge that each victim poses. Host diversity is a barrier to the spread of the disease, a barrier that can only be overcome with great difficulty. Indeed, some people, to judge from Francis Plummer's discovery of the antibody‐ negative Kenyan prostitutes, seem to be completely resistant to attack by the virus. Why should there be so much diversity among us, and what does this tell us about the way that we have evolved along with our diseases?
In addressing these questions we will confront the most astonishing story of all. So far in this book we have dealt with day-to-day skirmishes between ourselves and our diseases. It is now time to back away and look at the whole battlefield. We will discover an endless war that has shaped not just the diversity of our own species but that of the entire living world.
A few years ago, the immunologist Douglas Green and I began to wonder about the evolutionary consequences of diseases. Our thinking led us through a maze of possibilities, and down quite a few garden paths. But gradually a fascinating pattern began to emerge. Disease resistance and susceptibility in our species is incredibly complicated, and many different genes are involved. None of us, luckily, is susceptible