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.
[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.
[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.
[1][8] The idea of a link between parasite infection and immune disorders was first suggested in 1968[13] before the advent of large scale DNA sequencing techniques.
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 rise in allergic diseases such as asthma and hay fever during the 20th century.
[14] The hygiene hypothesis has also been expanded beyond allergies, and is also studied in the context of a broader range of conditions affected by the immune system, particularly inflammatory diseases.
[12] In 2003, Graham Rook proposed the "old friends" hypothesis which has been described as a more rational explanation for the link between microbial exposure and inflammatory disorders.
[20] The hypothesis states 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.
[citation needed] 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".
[21] The rise of autoimmune diseases and acute lymphoblastic leukemia in young people in the developed world was linked to the hygiene hypothesis.
[21] Although the idea that exposure to certain infections may decrease the risk of allergy is not new, Strachan was one of the first to formally propose it, in an article published in the British Medical Journal in 1989.
[citation needed] The "old friends hypothesis" proposed in 2003[20] may offer a better explanation for the link between microbial exposure and inflammatory diseases.
The organisms that are implicated in the hygiene hypothesis are not proven to cause the disease prevalence, however there are sufficient data on lactobacilli, saprophytic environment mycobacteria, and helminths and their association.
Exposure to diverse organisms in early development builds a "database" that allows the immune system to identify harmful agents and normalize once the danger is eliminated.
[citation needed] For allergic disease, the most important times for exposure are: early in development; later during pregnancy; and the first few days or months of infancy.
For instance, in very early human societies, small interaction between its members has given particular selection to a relatively limited group of pathogens that had high transmission rates.
[Figure 1Bach] However, the North South Gradient seen in the prevalence of multiple sclerosis has been found to be inversely related to the global distribution of parasitic infection.
[Fleming] Parasite infection has also been shown to improve inflammatory bowel disease and may act in a similar fashion as it does in multiple sclerosis.
Factors that favor a predominantly TH1 phenotype include: older siblings, large family size, early day care attendance, infection (TB, measles, or hepatitis), rural living, or contact with animals.
A TH2-dominated phenotype is associated with high antibiotic use, western lifestyle, urban environment, diet, and sensitivity to dust mites and cockroaches.
[44] There is a significant amount of evidence supporting the idea that microbial exposure is linked to allergies or other conditions,[2][6][7] although scientific disagreement still exists.
Helminthic therapy emerged from the search for reasons why the incidence of immunological disorders and autoimmune diseases correlates with the level of industrial development.
[12] 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 SARS and Ebola.
The idea that homes can be made “sterile” through excessive cleanliness is implausible, and the evidence shows that after cleaning, microbes are quickly replaced by dust and air from outdoors, by shedding from the body and other living things, as well as from food.
A key aspect of this approach is that it maximises protection against pathogens and infection, but is more relaxed about visible cleanliness in order to sustain normal exposure to other human, animal and environmental microbes.