Spend time outdoors, in teeming ecosystems like forests and oceans. The greater the diversity in the outside world you travel in, the greater the diversity in your gut, says Leach. If you have to be indoors, open the window and let the bacteria in the air blow in. Leach calls this the "rewilding" of your microbiome.

Gardening provides healthy hands-on, hands-dirty contact with the microbes in the soil, as does having a pet.

Exposure to the bugs that ride on cats and dogs may build the diversity of your own microbiota. People who are raised on farms, for example, are less prone to autoimmune disorders.