What is the “microbiome”? The word refers to the community of microbes, i.e., tiny organisms, that live in and on our bodies. This includes bacteria, viruses, certain fungi, and archaea (primitive single-celled organisms). In this book, I also count parasites among the microbes, although some of them are quite large. The term “microbiome” has become common in everyday language, although its meaning is not entirely clear-cut and the word is used differently depending on the case. The term “biome” in biology refers to the organisms of a habitat, so “microbiome” can refer to the entirety of the microorganisms of a body. This is the sense in which this book uses the word microbiome. We also use the well-established term “flora” (i.e., skin flora, gut flora, etc.) in the same sense, although it is actually a botanical term and the microorganisms discussed here are not plants!