My kid is a dirty skip rat and so were his brothers and none of them have had a day's illness (appendicitis excepted but that was not due to infection that was another story). I keep a tidy house but I don't believe in sterilising everything. My babies stopped having bottles sterilised as soon as they started reaching for stuff, about 3 months. I kept them clean but not sterile as the health visitor would have wanted it and it's paid off, their immune systems are robust and they are healthy.