Can a childhood that is too clean cause allergies?

The hygiene hypothesis suggests that a childhood with too little exposure to the right microbes may leave the immune system poorly tuned, which could raise the risk of allergies. Its standard statement is that early childhood exposure to particular microorganisms, such as gut flora and helminth parasites, protects against allergies by properly tuning the immune system. It is a hypothesis, not a settled fact: there is a significant amount of evidence for it, yet many scientists still reject it.

It began with an observation. In 1989 the British doctor David Strachan noticed that hay fever and eczema were less common in children from larger families, who were presumably exposed to more infections through their siblings. He proposed that lower rates of infection in early childhood could explain the rise in allergic diseases like asthma and hay fever during the 20th century. Until then, the increase had been put down to pollution.

The name is the biggest misunderstanding. It is described as a misnomer because people read it as being about personal cleanliness. In fact, worse personal hygiene, such as not washing hands before eating, only increases the risk of infection without affecting the risk of allergies. And the hypothesis does not suggest that having more infections in childhood would be an overall benefit.

A more careful version came in 2003, when Graham Rook proposed the "old friends" hypothesis. It says the vital exposures are not recent childhood infections like colds or measles but microbes that were already present during mammalian and human evolution. Sanitation, clean water and garbage collection cut exposure to cholera and typhoid, but they may also have deprived people of exposure to those old friends.

So the question is not whether clean water was a good idea, but which microbes the immune system actually needs to meet.