Being overly clean leads to allergies, many people surmise, because low-level dust/pollen/allergens are never properly introduced to develop the immune system. So, there's a difference between drinking dirty water and being OCD around the house. Clearly, we want surgeons to maintain sterile operating theatres. Its not clear we need sterile environments in every walk of life (where the skin is not broken, and the immune system not already compromised...etc).