The Guiness Book of Records and the Hitch Hikers Guide to the Galaxy are both books that document many astounding facts concerning life, the universe, and everything else. However, as far as I am aware, in neither of these eminent publications is there any mention of two truly amazing observations: an adult human being consists of ten times as many microbial cells as mammalian cells, and he or she carries around approximately 1.25 kg of microbes. Knowing this, who could fail to be intrigued by the microbial component of that mammal–microbe symbiosis known to us as a “human being.” The immediate questions prompted by this knowledge are usually along the lines of: (1) Which microbes are present? (2) How do they manage to survive? (3) What are they doing there? (4) Are they dangerous? Each of these questions about our “indigenous microbiota” is addressed in this book, and some of the answers will be surprising.

The complexity of the microbial communities found at many body sites is truly astounding; it has been estimated that we provide a home for at least 1,500 different microbial taxa that collectively contain more than 200 times as many genes as the human genome. Most of these organisms have not yet been grown in the laboratory, and so we know very little about them. However, modern molecular approaches are not only enabling us to detect their presence but, thanks to gene sequencing, also to gain some idea of their physiology and virulence potential.