You share your body with more microbial cells than you might expect

Yong opens by dismantling the intuitive picture of a human body as a single, self-contained organism: trillions of bacteria, along with viruses, fungi, and other microbes, live on and inside every person, concentrated especially in the gut, skin, and mucous membranes, collectively known as the microbiome.

He's careful to correct an often-repeated but overstated claim that microbial cells outnumber human cells by ten to one — more careful recent counts suggest the ratio is closer to roughly even — while still emphasizing that the microbiome's genetic and metabolic contribution to the body is enormous regardless of the exact cell-count ratio, since microbes carry far more total genes than the human genome itself.

Yong uses this recalibration to set up the book's central reframing: a human body is better understood not as a single organism that occasionally hosts germs, but as a complex ecosystem in which human and microbial cells are constantly interacting, cooperating, and co-shaping each other's function.

Takeaway: the precise cell-count ratio matters less than the reality that your body's genetic and metabolic capacity is massively expanded by the microbes living in and on you.