Bacteria in your body outnumber your own cells ten to one
The core idea stands, but key numbers or details have changed.
TaughtYou are only 10 percent human. Microbial cells outnumber your own cells by ten to one, a striking statistic repeated in every biology and health class.
NowThe ratio is closer to 1.3 to 1, and it fluctuates around 1 to 1 depending on when you last used the bathroom. A single bowel movement can shift it. You are roughly half human by cell count, which is still remarkable but not the same claim.
What actually happened
The 10:1 figure traces to a 1972 back-of-envelope estimate by Thomas Luckey, which was a reasonable guess for its time and then got cited for forty years without anyone rechecking the arithmetic. Dwayne Savage's 1977 paper repeated it, and from there it entered every textbook and TED talk about the microbiome.
Ron Sender, Shai Fuchs, and Ron Milo redid it properly in 2016 in PLOS Biology. Counting carefully, a reference adult has around 30 trillion human cells, dominated by red blood cells, which the old estimate had badly undercounted, and around 38 trillion bacteria, almost all of them in the colon. That gives about 1.3 to 1, with real uncertainty around it.
By mass, bacteria are only about 0.2 kilograms, because they are tiny. Genetically the microbiome still dwarfs us, carrying vastly more genes than the human genome, so the underlying insight that we are ecosystems rather than individuals survives intact. What died is the specific number, and it is a good lesson in how a single unchecked estimate can become a fact through pure repetition.
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Who was taught this
Still standard through 2016, so anyone who finished school between 1950 and 2016 learned the earlier version.