For most of the last fifty years, a striking claim appeared in textbooks and popular science: only about one in every ten cells in your body is actually yours. The other nine were said to be bacteria, riding along in your gut, on your skin and everywhere in between. It made a human being sound less like an individual and more like a walking colony.
The figure was wrong, and the way it got fixed is almost as interesting as the number itself.
In 2016 three researchers at the Weizmann Institute in Israel, Ron Sender, Shai Fuchs and Ron Milo, sat down to check the famous ten-to-one ratio properly. They tallied the best available estimates for both sides of the ledger, cell type by cell type. Their conclusion: a typical adult carries roughly 38 trillion bacteria and about 30 trillion human cells. That is nowhere near ten to one. It comes out close to one to one, near enough that a single large bowel movement can tip you briefly back into the human majority.
The 30 trillion figure hides a surprise of its own. The overwhelming majority of your cells are not muscle, bone or brain. They are red blood cells, tiny and countless, which make up more than eighty per cent of the total. The cells you might think of as the important ones are almost a rounding error by count.
So where did the old ten-to-one claim come from? Sender and his colleagues traced it to a single 1972 paper, in which a microbiologist named Thomas Luckey made a rough back-of-the-envelope estimate. It was never meant as a precise measurement. But it was a clean, memorable number, so it was quoted, and then the quotes were quoted, until a casual guess had hardened into a fact that everyone knew and nobody had rechecked for forty years.
The corrected ratio does not really change the headline. You are still host to tens of trillions of microbes, and they matter enormously to your health. What changed is the reminder underneath: a number can travel the whole world and settle into every textbook without anyone stopping to confirm it was ever true.
