On Oct. 9, 1676, Antonie van Leeuwenhoek reported the the discovery of bacteria in a 72 word letter to the Royal Society.

Van Leeuwenhoek described miniscule creatures living in rainwater: “animals that were ten thousand times smaller” than the water flea. For the first time in human history, he had observed single-celled microorganisms, a previously unimaginable form of life.

Keeping his tools and methods secret prevented others from replicating his observations, so his findings were not taken entirely seriously at first. It would be another year before others were able to observe the animals as well. Only then did a wonderful new world open up for research. Bacteria and protozoa became as real as rabbits or beeches.