Professionally, as a researcher (or former researcher, since I don’t do primary research anymore) in infectious disease, I have long been a proponent of the “hygiene” theory of immune-related diseases.  We are just too clean. 

Don’t challenge our respiratory systems at a young enough age?  We set ourselves up for asthma and allergies.

Don’t get enough intestinal diseases?  Crohn’s and irritable bowel.

These are disorders of industrialized nations.  We simply don’t see as much of them in developing countries.

Well, the rats agree with me.  Lab rats are immunological wimps.

Yep.  I let my kid play in the dog water.  It’s good for him.