Where did they come from and "What would happen if a new born was never inoculated with junk and studied for "x" amount of years to test this notion out about microbes -- when, where or how were they introduced?" would probably answer that question, or alteast give some very good hints.
But I do not doubt that that isolated-test baby which have lived a happy and un-junked life (?) have a stronger immune system. Simply like that. I am beginning to think that many small things alltogether bring the immune system down and we become suspectible to critters. Just like bugs will start to eat on a "sick"/poor flower?
Or maybe that is just too simple?