Allergies

It is noticeable that people in the developed world are more prone to allergies. As my mother-in-law used to say, "eat a peck of dirt before you die". 

It looks like she was correct with far fewer allergies with people like the Amish who live closer to horses and are more exposed to microbes. 

It would seem that we build up immunity by being exposed to these microbes which we are exposed to far less these days.