From The New York Times:
Scientists have long known that a little dirt can be good for you. Research has suggested that people who grow up on farms, for instance, have lower rates of Crohn’s disease, asthma and allergies, likely because of their exposure to a diverse array of microbes.
In the 1970s, scientists even found a soil-dwelling bacterium, called Mycobacterium vaccae, that has an anti-inflammatory effect on our brains, possibly both lowering stress and improving our immune response to it. More recently, there’s been an explosion of interest in the human microbiome — with people taking probiotics, seeking food with live cultures and “rewilding” their microflora. At the same time, scientists have been discovering how broad a role dirt microbes can play in our mental and physical health.