Parents are constantly being told to make their kitchens spotless, to kill 99.9 per cent of the germs lurking in their bathrooms and to wash themselves and their babies all the time.
This world of purity sounds good but it does not fit how we are designed. We are meant to encounter some microbes and dirt when we are young. It is how we built our immune systems. We need a certain amount of grunginess as kids to be healthy adults.
As the Harvard study shows, filth can be good -- at least in tiny amounts when you are very young.
Image: Taking a child's temperature, via Shutterstock