Hygiene hypothesis

In medicine, the hygiene hypothesis states that early childhood exposure to particular microorganisms (such as the gut flora and helminth parasites) protects against allergies by properly tuning the immune system.^[1][2] In particular, a lack of such exposure is thought to lead to poor immune tolerance.^[1] The time period for exposure begins before birth and ends at school age.^[3]

While early versions of the hypothesis referred to microorganism exposure in general, later versions apply to a specific set of microbes that have co-evolved with humans.^[1][4][2] The updates have been given various names, including the microbiome depletion hypothesis, the microflora hypothesis, and the "old friends" hypothesis.^[4][5] There is a significant amount of evidence supporting the idea that lack of exposure to these microbes is linked to allergies or other conditions,^[2][6][7] although it is still rejected by many scientists.^[4][8][9]

The term "hygiene hypothesis" has been described as a misnomer because people incorrectly interpret it as referring to their own cleanliness.^{[1][8][10][11]} Having worse personal hygiene, such as not washing hands before eating, only increases the risk of infection without affecting the risk of allergies or immune disorders.^[1][4][9] Hygiene is essential for protecting vulnerable populations such as the elderly from infections, preventing the spread of antibiotic resistance, and combating emerging infectious diseases such as Ebola or COVID-19.^[12] The hygiene hypothesis does not suggest that having more infections during childhood would be an overall benefit.^[1][8]