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 allergicdiseases by contributing to the development of the immune system.
In particular, a lack of exposure is thought to lead to defects in the establishment of immune tolerance. The time period for exposure begins in utero and ends at school age.
While early versions of the hypothesis referred to exposure to microbes in general, updated versions apply to a specific set of microbial species that have co-evolved with humans.
The updates have been given various names, including the microbiome depletion hypothesis, the microflora hypothesis, and the "old friends" hypothesis.
There is a significant amount of evidence supporting the idea that lack of exposure to these microbes is linked to allergies or other conditions, although scientific disagreement still exists.
The term "hygiene hypothesis" has been described as a misnomer because people incorrectly interpret it as referring to personal cleanliness.
Reducing personal hygiene, such as not washing hands before eating, is expected to simply increase the risk of infection without having any impact on allergies or immune disorders.
Hygiene is essential for protecting vulnerable populations such as the elderly from infections, preventing the spread of antibiotic resistance, and for combating emerging infectious diseases such as Ebola or COVID-19.
The hygiene hypothesis does not suggest that having more infections during childhood would be an overall benefit.
Children growing up in rural areas, around animals and in larger families seem to develop asthma less often than do other children.
According to the hygiene hypothesis, this is due to increased exposure to particular viruses, bacteria or parasites.
The hygiene hypothesis proposes that childhood exposure to germs and certain infections helps the immune system develop. This teaches the body to differentiate harmless substances from the harmful substances that trigger asthma.
In theory, exposure to certain germs teaches the immune system not to overreact.
But preventing asthma isn't as simple as avoiding antibacterial soap, having a big family or spending time on the farm.
Current research suggests that this potentially helpful exposure to certain germs may actually occur before a baby is born, when he or she is still in the womb.
Researchers are discovering that a pregnant mother's exposure to infectious germs or other substances, such as vaccines, may play an important role in the development of a baby's immune system and gut microbiome.
For now, more research is needed to understand exactly how childhood germ exposure might help prevent asthma.