Finnish researchers have conducted an interesting study of where people who suffer from common respiratory allergies live. They found that people who live in rural areas are significantly less likely to suffer from allergies than those who live in large towns. They related this to the populations of plants in the community and of bacteria on the skins of their subjects; rural residents not only have more diverse communities of plants and animals around their homes, they have more diverse bacterial populations on their skins. Lifelong exposure to wild plant communities and rural bacteria seems to provide some protection against allergies.
The effect is not all that large, though, and allergy sufferers are found in all environments. But this might be one reason why the number of allergy sufferers keeps increasing -- the neater our neighborhoods and the less diverse the bacteria on our skins, the more likely we are to sneeze.