Although Stu's tongue in cheek humour up top was exactly that I think it is a genuine point. When we are born we have effectively no proper immune system, our bodies effectively learn what is good and what is bad. Some of that is genetics but some of it is interaction with the environment. Kids nowadays are locked up in super clean environments, going outside less, have access to anti-biotics on tap etc. I remember reading up on asthma when I was younger when my younger brother developed it, interesting story of a scientist looking at the fall of the Berlin wall and studying populations from cities on either side. They tested for asthma in populations in east and west Germany, the eastern city was Liepzig I think which is industrial full of polluting factories but despite the pollution had lower rates of asthma compared to Munich.
Funny anecdote - if anyone has watched a sitcom called Early Doors, the old cantankerous guy on it interjects when some of the characters are talking about organic food. He says it all used to be organic in the old days, then they charged you more to put preservatives in to make it last longer, now they are taking it out they are charging you lot even more again!