The more you think about sickness and health, the trickier it gets to draw a clean line between them. We tend to think of ourselves as being prepared by nature for a good life. If we can just keep bacteria and viruses from killing us, and avoid walking into open elevator shafts, we’ll live a long, healthy life.

But we are actually the products of evolution, and evolution can’t give us perfect health. It has endowed us with powerful immune systems, thank you very much. And it has endowed us with quick reflexes that can, in some cases, keep us out of open elevator shafts. But evolution doesn’t automatically march to perfection. It stops short, leaving us with grave imperfections.

We have lots of