A gut full of diverse microbes ? bacteria, viruses and fungi ? is essential for a healthy mind and body. And evidence is growing that our modern diet, overuse of antibiotics and obsession with cleanliness are damaging the diversity of microbes that live in our guts, contributing to a range of conditions including depression, multiple sclerosis, obesity and rheumatoid arthritis. Microbes live in our guts, bodily fluids, cavities and skin. For every one of our human cells, there?s at least one of them.