A recent study suggests that the dirtier you are, the better your resistance towards allergens that can trigger asthmatic attacks.
from Reuters Health, 21 April 2009
'Dirty' here refers to lice in your head, and it is assumed that if you have legions of lice taking residence, then you are not the type who probably believes in cleanliness being next to godliness.
Our immune systems need to be challenged early (read: you have to get sick when you're young) because that is the best way it exerts its function. Between getting sick naturally and developing antibodies from vaccines, the former is always better than the latter.
Lesson: A hyper-clean body might mean a weaker immune system.