We have known for centuries that we share our bodies with microbes. But we always assumed they lived in places, like skin, mouth, and gastrointestinal tract, which are “outside” our bodies, and that they just use us for housing. Living on us but not interacting with us. However, new findings from the Human Microbiome Project and other lines of study have strongly suggested that our relationships with our microbes are much closer than we thought. Much closer. Who, or what, are we? DNA studies of human tissue have revealed that, at least in number of cells, there are nearly as many of  “them” as “us”. But they are too small for us to see them. But, for instance, about 90% of the cells in the gut are microbes! Not only that, but microbes seem to inhabit every tissue of our bodies, albeit in small numbers (at least compared to skin and guts). And yes. This includes the brain. What are they doing there?   Most of our microbes are in our guts.   There are …