There are a lot of bacteria on us and in us — our microbiome, it is called. Calculating exactly how many microbes each of us carries is hard, and the most common number cited, both in popular and scientific literature, is almost certainly wrong.

In 1972, Thomas D. Luckey published an article in The American Journal of Clinical Nutrition in which he wrote that an adult male had 100 x 1012, or 100 trillion, microbes in his gut, and another one trillion on his skin. He gave no reference for either figure.

Another scientist, D. C. Savage, cited Dr. Luckey in a review published in 1977, writing that “the normal human organism can be said to be composed of over 1014 cells, of which only about 10 percent are animal cells.” So, he used Dr. Luckey’s number, 100 trillion, for bacteria, and added that one-10th of those — 10 trillion — were human cells.

This neat ratio caught on, and the phrase “10 times as many microbial cells as human cells” is often repeated, as it is on the website of the Human Microbiome Project of the National Institutes of Health, where presumably they know something about the microbiome.