The world is full of microscopic creatures that can make us sick, and we just take that for granted. If your friend has COVID and coughs on you, you might get COVID. If you eat food you dropped on the ground (more than five seconds ago), it might make you sick. And if you go in for surgery, your surgeon better Wash their hands really well.
But it took a long freaking time for this concept to be dreamed up in the first place, much less accepted. The concept of dirty or rotten things being bad for you was a well-known phenomenon, but the explanations didn’t get at the core idea of there being bad things that could make you sick in there. Instead, there were ideas like staying away from bad smells, or huffing perfume-soaked rags to keep disease away. Even when death rates plummeted after a hand-washing policy was implemented in a maternity hospital in 1846, the idea didn’t really take off. The guy who invented a microscope and said he saw “animalcules” swimming around in Semen and tooth plaque was ridiculed.
Finally, Louis Pasteur’s work in the 1860’s started to change enough Minds that germ theory became a thing. Diseases started to be linked to particular bacteria, with one of the first notable cases being the Discovery of the Anthrax Bacillus in 1876.