In the movie Princess Bride, Westley took an ever-increase dose of poison over a long period of time to build up his resistance, part of his herculean effort to rescue the princess.
A recent Darwinian award winner tried the same with cyanide and died.
Scientists seem to agree with Westley on the matter of peanut butter; they proved that kids can indeed build up their immunity by eating very small amount of peanuts to start.
When I was a kid, adults frequently said “不乾不淨,吃了沒病” (a little insanitarity makes you healthier). I never quite understood it. Then I found American kids are allergic to everything. Could it be, gosh, that they grew up too cleanly?
Either that, or they really should let natural selection work harder.