Antonie van Leeuwenhoek came up with “animalcules”. As you may have guessed, it refers to tiny animals. Which tiny animals exactly?
Before I can answer that question, I must tell you about Antonie. He must be someone special, because he has a Google Doodle. He was a draper from the Dutch city of Delft. Drapers frequently used magnifying glasses to examine the quality of their fabric. Dear Antonie was not satisfied with the quality of his lens, so he taught himself lens making.
How many of us could do that? And he did it in 1671! He had no modern tools to fashion his lens or make the apparatus to hold it (see the image), AND he made all of it the size of a flash drive! On top of that, he managed to keep his lens production method a secret. We still don’t know exactly how he polished his lenses.

van Leeuwenhoek’s Microscope
1 – sample holder
2 – tilt
3 – lens
4 – depth (closeness to lens)
Antonie was a merchant without much funds for this project and was not schooled in the sciences, yet he made a lens that was so good he saw things that no one else could see. A truly talented man!

He looked at red blood cells, rotifers, sperm, daphnia–and you say wait, did you say sperm? Animal sperm folks, animal. And now you’re thinking, “How the heck did he–” Nope, not going there.
Antonie’s animalcules are now known as microbes. Personally, I think we should have kept “animalcules”. He drew everything he saw. He drew a lot because he looked at everything in his world.

The blood cells could have been his own. When I had my first ‘scope, I looked at my blood and my dog’s (I’ll share that story another time).
He was not a member, or fellow, of the Royal Society. That didn’t stop him. He sent letters and sought publication with them. At first he wasn’t believed, and he was required to undergo interviews with a vicar and others chosen by the Royal Society to prove he was of sound mind. Then he had to demonstrate the workings of his microscope. Once he was finally believed, scientists had a window into a new world. He became a Foreign Fellow of the Society in 1680.
He has been dubbed the Father of Microbiology.
Watch a fabulous 6 min. video produced by the Howard Hughes Medical Institute