The Hygiene Hypothesis
The "Hygiene Hypothesis" is a theory that suggests a young child's environment can be "too clean" to effectively stimulate or challenge the child's immune system to respond to various threats during the time a child's immune system is maturing. As a fetus, the immune system is thought to be repressed to avoid rejecting maternal tissue, but at birth, it must start to recognize antigens that may be linked to harmful infections. If the environment is "too clean," the hypothesis suggests that the immune system will not mature properly, and may not react appropriately when the child's immune system encounters germs (viruses, bacteria, fungi, and parasites) or other environmental triggers (pollen, animal dander, fungal spores) later in life. The hypothesis suggests that the lack of immune system challenge results in many people developing immune-related health problems such as asthma.
Although the term "Hygiene Hypothesis" was proposed in the late 1980's, many investigators trace its origins to earlier experiments with animals, mainly mice and rats born and raised in "germ-free" or sterile environments. When experimentally exposed to low doses of an infectious agent that would not harm normally raised rodents, these germ-free adolescent and young adult animals became infected quickly and often died. When examined, the infected animals showed a slow or blunted immune response in these situations. If they were raised to adolescence or adulthood as germ-free animals and then were slowly introduced to bacteria found in their normal gastrointestinal tracts, the rodent's immune response, when exposed to the same pathogen, was much better and the animals usually survived the infection.
The key to understanding how the hygiene hypothesis might explain the high level of asthma in developed nations (and perhaps a number of other diseases such as Crohn's disease, ulcerative colitis, or multiple sclerosis) is knowledge of how the immune system develops, matures, and self-regulates. Although the details of the development and function of the immune system can fill books, a very brief summation is as follows: