hygiene hypothesis
n. The theory that a lack of early childhood exposure to dirt, bacteria and other infection-causing agents can lead to immune system weakness and a increased risk of developing allergies and asthma.
The new findings, published today in The New England Journal of Medicine, add to a growing collection of evidence for the "hygiene hypothesis." This theory suggests that 20th century advances like indoor plumbing, antibiotics and cleaner homes may have contributed to recent increases in allergy, asthma and eczema by decreasing rates of childhood infection. Some infections early in life, the argument goes, help the immune system develop properly.
—Denise Grady, “Environment Rich in Germs May Reduce Risk of Asthma,” The New York Times, September 19, 2002
1995 (earliest)
The hygiene hypothesis attributes the rise in appendicitis that occurred in the United Kingdom at the beginning of this century to improvements in sewage disposal and water supplies in the late 19th century. 2 These improvements in hygiene greatly reduced the exposure of infants to enteric organisms that programme the immune system of the gut, thereby rendering the bowel more susceptible to triggering infection later in life.
—Alfredo Pisacane, et al., “Breast feeding and acute appendicitis,” The British Medical Journal, April 01, 1995
According to the American Academy of Allergy Asthma and Immunology, the number of people with asthma increased by 75 percent from 1980 to 1994. From 1985 to 2000, the number of people with some kind of allergy doubled. Why? If you believe the hygiene hypothesis, it's because of our society's obsession with creating germ-free environments for our kids.

The immune system is a self-improving mechanism that fights off bacteria and other foreign intruders by creating antibodies specifically designed to disable those intruders. These antibodies then become a full-time part of the immune system, enabling the body to more effectively repulse future invasions by the same foreign substance.

Therefore, so the theory goes, the more germ-free an environment is, the fewer antibodies a kid's immune system will create. This not only leads to an overall weakness in the immune system, but it also means the immune responses will often be downright strange, such as skirmishing with normally harmless substances like cat dander, pollen, and peanuts.
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