On second thought ...

... Diet, gut microbiota and immune responses : Nature Immunology : Nature Publishing Group.

The hygiene hypothesis is now the prevailing explanation for the increase in asthma and atopic disorders in Western countries. It suggests that excess cleanliness in the environment has led to a decrease in the number of infectious stimuli needed for proper development of the immune system. The prevalence of asthma and allergies is lower for people who are raised on a farm and those who belong to larger families or are in a lower birth order in such families and is related to socioeconomic status.