We have way more bacteria than we think we do. Each of us carries around about 10 times more bacteria cells than our own human cells. And, these bacteria are everywhere. They are on our skin, they are in our GI tract, and in our respiratory tract.
Generally we tend to think of bacteria with an uneasy sense of foreboding and disapproval. But there are actually more good bacteria than bad ones. The good ones are known as commensal bacteria and they can do a nice job of stimulating your immune system.
"Studies by other researchers have found that mice raised in sterile, germ-free environments have poorly developed immune systems. But until now, scientists have not known the mechanism by which bacteria help the immune system."
This research was performed by Loyola University Health System. Katherine Knight, Ph.D., a professor and chair of the Department of Microbiology and Immunology at Loyola University Chicago Stritch School of Medicine, was senior author of the study.
http://www.medicalnewstoday.com/articles/191826.php