ABSTRACT
Disease is often what comes to mind when considering the presence of microbes inside the human body. This is not surprising, given the tight coupling of the discovery of microbes with germ theory by Robert Koch and Louis Pasteur and the ensuing focus on the pathogenicity of microbes in both the scientific and popular literature. Infectious diseases remain a huge global health burden (Dye 2014) and a salient aspect of day-to-day life. Often overlooked are the commensal and even mutualistic interactions of microbes and humans. However, these interactions are not just important to human health; they are, in fact, fundamental to what it means to be human. This is because our bodies harbor approximately as many microbial cells as human cells, mainly in the gut (Sender, Fuchs, and Milo 2016).
