The hygiene hypothesis is a theory within the medical community which suggests that humans may actually be living lives which are too clean for their own good. The argument of the hygiene hypothesis is that early childhood exposure to things like bacteria, parasites, and so forth may teach the immune system to recognize these things, allowing it to focus on its original purpose, protecting the body from disease. Lack of such exposure may potentially be behind rising allergy rates in the developed world, according to the hygiene hypothesis.
This concept was originally proposed by a British researcher, David Strachan, in 1989. Strachan looked at the health of large families as opposed to small ones, and discovered that in families with many children, the children were often healthier and less prone to allergies. Strachan believed that this might be connected to increased exposure to things like bacteria which is common to large families, as it becomes difficult to control exposure to diseases when a large group of children is involved.
Researchers have also looked at other trends in modern human life which minimize exposure to harmful organisms, such as increased antibiotic use and the use of antimicrobial cleaning agents in the home. Some also believe that the development of things like airtight doors and windows has contributed to an accumulation of allergens in the home, by trapping these things indoors, rather than allowing them to circulate out.
The implication is that people who are exposed to potentially harmful organisms will develop an immune system which is capable of fighting such organisms, potentially making someone hardier. According to the hygiene hypothesis, when the immune system is not occupied with things like developing ways to fight parasites, it may learn to attack random foreign bodies like pollen, pet dander, and so forth. Essentially, by living “too cleanly,” people may be hampering the development of their immune systems.
Essentially, exposure to harmful things helps the immune system to regulate itself. It develops special cells known as t-cells which fight disease, and these cells learn to identify harmful substances only through exposure. Without being exposed to the myriad of organisms in the world which attack the human body, the immune system has no frame of reference, and instead it attacks things without any sort of checks or controls, potentially leading to various autoimmune diseases, among other things.
Several studies have been conducted on the hygiene hypothesis, and there is some evidence to support it. This does not mean that you should expose your children to a plethora of harmful bacteria and parasites, but it does mean that eating a little dirt might not necessarily be harmful. It may also be helpful to ease off on drugs like antibiotics, allowing the body to learn to fight mild infections on its own.