The rapid rise in food allergies, asthma and other immunological diseases is due, at least in part, to our modern obsession with cleanliness, scientists increasingly believe.
The 'hygiene hypothesis', first advanced in 1989 by the British epidemiologist David Strachan, contends that these diseases are becoming more common because young children are not exposed to them at an early age.
We spend so effort trying to prevent exposure to germs with antibiotics, antibacterials and soaps that letting kids get dirty seems like a violation of basic parental duty.
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.