The human gut microbiome contains hundreds to thousands of bacterial species, archaea, fungi, and viruses.

“And all those microbial cells, if you start to get to know them, and see who they are, break out in the gut probably to hundreds, to 1,000 species, depending upon how you define microbial species. And then most of these are bacteria, but there are a lot of other life forms there. There are archaea, which are little microbes that are bacterial-like, but they're different. There are eukaryotes. So we commonly think of eukaryotes in the gut as something like a parasite. But there are eukaryotes, there are fungi, there are also little viruses. There are these bacteriophages that infect bacterial cells.”