A collective term for organisms too small to see with the naked eye and observable only under a microscope, including bacteria, archaea, protists, and some fungi and algae.
Textbooks taught for decades that bacteria outnumber human cells 10 to 1 — a 1972 rough estimate cited without scrutiny. A 2016 Weizmann study found about 38 trillion gut microbes vs. 30 trillion human cells: 1.3 to 1, not 10 to 1. The number was inflated tenfold.