This is fascinating. It makes sense, that it should be more important to survive pathogens than to be clever, but the effect on so many things is remarkable.
Who'd have thought that on part of the immune system, an anti-inflammatory, interleukin-6 (IL6), should have such a powerful effect on personality, with high-levels of conscientiousness, openness and neuroticism all more common where disease burdens are lower.
It also makes sense that optimism should be linked with a lower disease burden.
I suppose it's less of an appealing set of results to those who put a high store on use being responsible, through 'free will', for the way we are.
It seems a bit perverse that dim, unadventurous, impulsive, bovine types should be more attractive and, thus, fecund, in places where there's a high level of infectious disease - but there you are, that's the way it is, because of IL6... at least that seems the most reasonable direction of causation for a genetic trend.
Still, IL6 doesn't seem to have any effect on agreeableness or extraversion, so, maybe these people are hail-fellow-well-met types to make up for their otherwise dull approach to life.
It's a very good reason to be careful in the use of anti-biotics and earnest in looking for replacements - we probably spend too much on cancer research compared to pathogen extermination research.
This also provides some support for David Horrobin's view that schizophrenia was related to the immune response and could be ameliorated by essential fatty acids in the diet. -- I've checked and Omega-3 does decrease IL6 levels significantly. ( http://www.ncbi.nlm.nih.gov/pubmed/26704762 )