It's possibly one of the things, but I doubt there's a single factor in play here. You could have a similar hypothesis about urban people having less contact wi

It's possibly one of the things, but I doubt there's a single factor in play here. You could have a similar hypothesis about urban people having less contact with dirt and mud and animal feces and all kinds of wild plants and insects that one living in a rural area would commonly encounter throughout her life. The other theory blames the lack of parasites that suppress our immunes system reactions. Some say it's the pollution in urban areas. And it's probably some sort of the combination of all of these, since they all are certainly a factor influencing our immune systems.