An empire of germs dwells inside you, trillions strong. About a half-pound of bacteria plus their genes make up our microbiome. Though each microbe is small, a healthy and diverse microbiome is mighty. Its influence, studies suggest, spans the human condition from mood swings to weight gain. The microbiome begins as a departing gift from mothers at birth, but many factors alter its composition. Growing evidence shows location has a profound impact on the diversity of microbes, and some places are much less diverse than others. A study published this week in the journal Cell follows multi-generation immigrants from Southeast Asia to the United States……..