You may find the hygiene hypothesis interesting - the theory that the incredibly sharp rise in allergic diseases in developed countries is due to no longer bein

You may find the hygiene hypothesis interesting - the theory that the incredibly sharp rise in allergic diseases in developed countries is due to no longer being exposed to the types of bacteria that we had during most of evolution. One example is that allergy rates in Finnish Karelia are much higher than right across the border with Russia, even though the populations and environment are otherwise very similar.