“Some researchers have proposed that the sharp rise in asthma and allergy cases over the past century stems, unexpectedly, from living too clean. The idea is that routine exposure to harmless microorganisms in the environment—soil bacteria, for instance—trains our immune systems to ignore benign molecules like pollen or the dandruff on a neighbor’s dog. Taking this “hygiene hypothesis” in an even more surprising direction, recent studies indicate that treatment with a specific soil bacterium, Mycobacterium vaccae, may be able to alleviate depression.”

[Note: This is from March 2007]