What is the hygiene hypothesis?
The hygiene hypothesis is a theory linking certain aspects of modern life - antibiotics, sanitation, clean drinking-water, and better hygiene - with less exposure to a diverse range of bacteria, viruses and other microorganisms. In effect, we may be making our environment too clean. This, some people say, may cause the body's immune system defences to develop poorly.
This theory has been studied in relation to a range of health conditions, including dementia, asthma, and allergies.
There have been conflicting results when the theory has been tested.
In 2012, claims that a rise in allergies is down to a modern preoccupation with cleanliness was dismissed as a myth by scientists from the London School of Hygiene and Tropical Medicine and University College London.
They raised concerns that people worrying about 'being too clean' might result in people needlessly exposing themselves and their children to things that could make them ill, which they said would be dangerous.
In 2016, a study of children in the journal Pediatrics found that those who suck their thumbs and bite their nails between the ages of 5 and 11 have a lower risk of developing allergies. This, they say, could expose them to more germs. However, they note that these habits can lead to dental problems and hand infections.