(From the "humans are crazy" and "truth is stranger than fiction" departments...)

Want to be happy?  Try eating dirt... or at least dirty plants.

Seriously.

From an article in Discover magazine, "Is Dirt The New Prozac?":

The results so far suggest that simply inhaling M. vaccae—you get a dose just by taking a walk in the wild or rooting around in the garden—could help elicit a jolly state of mind. “You can also ingest mycobacteria either through water sources or through eating plants—lettuce that you pick from the garden, or carrots,” Lowry says.

Graham Rook, an immunologist at University College London and a coauthor of the paper, adds that depression itself may be in part an inflammatory disorder. By triggering the production of immune cells that curb the inflammatory reaction typical of allergies, M. vaccae may ease that inflammation and hence depression. Therapy with M. vaccae—or with drugs based on the bacterium’s molecular components—might someday be used to treat depression. “It’s not clear to me whether the way ahead will be drugs that circumvent the use of these bugs,” Rook says, “or whether it will be easier to say, ‘The hell with it, let’s use the bugs.’”

Given the way the industry works, we'll probably either see drugs, or somebody will patent the bacteria.  But that's sort of secondary.  The real point is that to the extent our current environment doesn't match our ancestral one, there are likely to be "bugs", no pun intended.

(The original study: “Identification of an Immune-Responsive Mesolimbocortical Serotonergic System: Potential Role in Regulation of Emotional Behavior,” by Christopher Lowry et al., published online on March 28 in Neuroscience.)