NSFW, because George Carlin:
The “hygiene hypothesis” posits that “for many children in the wealthy world, a lack of exposure to bacteria, viruses, and allergens prevents the normal development of the immune system, ultimately increasing the chance of disorders within this system down the road”:
“A child’s immune system needs education, just like any other growing organ in the human body,” says Erika von Mutius, a pediatric allergist at the University of Munich and one of the first doctors to research the idea. “The hygiene hypothesis suggests that early life exposure to microbes helps in the education of an infant’s developing immune system.” Without this education, your immune system may be more prone to attacking the wrong target — in the case of autoimmune diseases, yourself.
It’s still a matter of active debate among scientists, but evidence for the idea has been slowly accumulating over time, both in humans and animal subjects. Most recently, a new study published earlier this month found that babies who grow up in houses with higher levels of certain bacteria — carried on cockroach, mouse, and cat dander — are less likely to develop wheezing and asthma by the age of three.
The Dish 