Hygiene hypothesis

In medicine, the hygiene hypothesis is a hypothesis that states a lack of early childhood exposure to infectious agents, symbiotic microorganisms (such as the gut flora or probiotics), and parasites increases susceptibility to allergic diseases by suppressing the natural development of the immune system. In particular, the lack of exposure is thought to lead to defects in the establishment of immune tolerance.

The hygiene hypothesis has also been called the "biome depletion theory" and the "lost friends theory".

The original formulation of the hygiene hypothesis dates from 1989 when David Strachan proposed that lower incidence of infection in early childhood could be an explanation for the rapid 20th century rise in allergic diseases such as asthma and hay fever.

It is now also recognised that the "reduced microbial exposure" concept applies to a much broader range of chronic inflammatory diseases than asthma and hay fever, which includes diseases such as type 1 diabetes and multiple sclerosis, and also some types of depression and cancer.

In 2003 Graham Rook proposed the "old friends hypothesis" which seems to offer a more rational explanation for the link between microbial exposure and inflammatory disorders. He argues that the vital microbial exposures are not colds, influenza, measles and other common childhood infections which have evolved relatively recently over the last 10,000 years, but rather the microbes already present during mammalian and human evolution, that could persist in small hunter gatherer groups as microbiota, tolerated latent infections or carrier states. He proposes that we have become so dependent on these "old friends" that our immune systems neither develop properly nor function properly without them.

Strachan's original formulation of the hygiene hypothesis also centred around the idea that smaller families provided insufficient microbial exposure partly because of less person-to-person spread of infections, but also because of "improved household amenities and higher standards of personal cleanliness". It seems likely that this was the reason he named it the "hygiene hypothesis". Although the "hygiene revolution" of the nineteenth and twentieth centuries may have been a major factor, it now seems more likely that, although public health measures such as sanitation, potable water and garbage collection were instrumental in reducing our exposure to cholera, typhoid and so on, they also deprived us of our exposure to the "old friends" that occupy the same environmental habitats.

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