The human body contains more bacterial cells than human cells

For decades, textbooks claimed the human body hosted 10 bacterial cells for every human cell. A 2016 revision by Ron Sender and colleagues at the Weizmann Institute put the ratio at closer to 1:1 — roughly 38 trillion bacteria to 30 trillion human cells. Either way, the human gut alone contains about 1,000 bacterial species and a combined genome (the microbiome) of over 3 million unique genes, dwarfing the human genome's 20,000. The microbiome influences immunity, mood, and metabolism in ways researchers are only beginning to understand.

Source: Sender et al., Cell (2016)