Schizophrenia, at least the inherited kind, is linked to greater
disease resistance. One book I read claimed that the reason it is so
common in European populations is the Black Plague, which left more
schizos alive than normals. Schizos are linked to intelligence as
well, the blood relatives of schizos tend to be above normal
intelligence. And how about Homosexuality? Seem to be unusually well
represented in any field that requires thinking originally, art,
science etc. A society might benefit from having a few people able to
look at things from new viewpoints. I suspect there are lots of these
things if we look.

For example, arthritis and hay fever are both symptoms of an over
reactive immune system. People with one rarely get the other, a small
blessing, because they represent over-reactions in opposite
directions, so to speak. I would be interested to know of any research
which link these to particular disease resitances. The human immune
system is particularly hair trigger, we are tougher than most other
animals, 10,000 years of steadily increasing concentration in squalid
cities. Read any history and it tells of plauge after plague wiping
cities clear every few years. The result is modern humans, able to
live in any environment and survive most diseases, but prone to immune
system overreactions like allergies and arthritis.