I don't wash my hands and never get sick. Any theories?
You may very well be strengthening your immune system, but there are microorganisms that you don't want to get, which can have longer term health consequences. For example, there seems to be a significant increase in developing MS if you've ever had the Epstein-Barr virus. But this is just one case. You also don't want Salmonella, tape worms, etc. 

So there may be a price to pay for a stronger immune system: namely higher probability of exposing yourself to the more dangerous things out there.