The Winter 2007-2008 issue of Organic Gardening (a ghost of what it used to be, unfortunately) has a blurb titled "Soil Makes You Happy":
Quote Proof that dirty fingers lead to clean minds. Findings: A common soil bacterium, Mycobacterium vaccae, is an effective vaccine for leprosy. Researches began to evaluate its value in treating asthma, tuberculosis, and cancer. When cancer patients treated with M. vaccae reported feeling inexplicably happier, neuroscientist Christopher Lowry, PhD., of the U.K.'s University of Bristol injected mice with the bacterium, then examined their brains. The mice's immune systems were stimulated, causing brain cells to release serotonin, a mood-altering, pleasure-inducing hormone. Our Advice: Dig in! Be happy!