George Carlin "...immune system is to kill germs. It needs germs to practice on ... When i was a little boy in 1949 we swam in Hudson river. The Hudson River was loaded with raw sewage. That's right, we swam in raw sewage. You know, to cool off. ...":

http://www.youtube.com/watch?v=CnmMNdiCz_s&list=PL4DDBE51446...

Edit: imagine, theoretically, if there is no germs and no need for immune system, would it mean additional biological resources available inside the human body for something else, like for bigger/better brain, etc...?