What is the hygiene hypothesis?

From ABC Health and Wellbeing, 28 March 2013:

It sounds counter-intuitive, but not being exposed to viruses, bacteria and parasites in your early years could have a negative effect on your health, or at least that’s what the hygiene hypothesis suggests.

The idea, which is subject to much debate, suggests our obsession with cleanliness and sterile surroundings is causing the increase in allergic and autoimmune diseases we’ve seen in recent decades.

Gastroenterologist Professor Timothy Florin says the term ‘hygiene hypothesis’ has been bandied around a lot, but has generally come to refer to the effect of children not being exposed to enough pathogens at an early age.

“The basis is that our immune system has evolved over evolutionary time as a defence against bacteria and viruses – bacteria and viruses have evolved over time with humans, rather like domestic dogs and pets, and they’re used to seeing each other,” says Florin, from the University of Queensland and Brisbane’s Mater Hospital.

“So we’re used to, in our growth as a population, seeing certain viruses at certain times and if you don’t see them at that time then it’s a bit like missing out on a maths lesson at a critical stage in infancy – you’re missing certain building blocks.”

Emerging hypothesis

The idea was first put forward in the 1980s by epidemiologist David Strachan, after he studied a cohort of more than 17,000 British children in an attempt to understand the gradual increase in incidence of hayfever in post-war Britain. During his research he noticed a striking pattern emerging.

The more older siblings a child had, the less likely he or she was to develop hay fever or eczema by age 23. He found it was by far and away the most important risk factor in determining whether a child developed the allergic condition.

Strachan concluded there must be some protective factor spread from older siblings to younger siblings.

“[These observations] could, however, be explained if allergic diseases were prevented by infection in early childhood, transmitted by unhygienic contact with older siblings, or acquired prenatally from a mother infected by contact with her older children,” he wrote in the British Medical Journal, on the 18 November, 1989.

Since then, other studies have found links between various aspects of Western living standards and a range of allergic diseases – including eczema, asthma, and food allergy – or autoimmune conditions such as type 1 diabetes.

Research also suggests children born to families migrating to western countries also experience an increase in the incidence allergic disease. And it’s been found certain factors – such as attending childcare or having a pet – can help to reduce the risk of allergic disease. Read more.