In the late 1600s, using his own, highly-magnifying microscopes, Leeuwenhoek observed tiny, moving organisms in water, which he called "animalcules". He described these organisms as having various shapes, including spheres (cocci), rods (bacilli), and spirals (spirilla).

While he didn't fully understand the significance of his discoveries, Leeuwenhoek's observations laid the foundation for the study of microbiology.