You carry roughly as many bacterial cells as human cells

For decades, textbooks claimed your body’s microbes outnumber your own cells ten to one. That figure traced back to a single rough estimate from the 1970s that was never meant to be definitive.

A careful 2016 recount put the numbers much closer. A 70-kg “reference” adult carries about 3.8 × 10¹³ bacteria and roughly 3.0 × 10¹³ human cells — a ratio near 1.3 to 1, not 10 to 1. Most of those human cells, around 90%, are tiny red blood cells.

All those bacteria together weigh only about 0.2 kg — less than half a pound.

So the headline isn’t quite “more microbe than human,” but it’s still striking: with each visit to the bathroom, you can briefly tip the balance back toward your own cells. You are, in effect, a walking community — a partnership of human and microbial life sharing one body.