Who discovered germs?
Well, there are several people we could point to, but I will give the credit to Anton von Leeuenhoek, a Dutch scientist who is generally given credit for inventing the microscope, among other things. Leeuenhoek was looking at a slice of cork under a microscope, and noticed the structure resembled a lot of boxes, all stacked together. He called these structures cells.
A couple of years later, Leeuenhoek was looking through his microscope again, this time at a sample of pond water. He noticed little cells swimming through the water, which he called "animalcules". He was probably seeing protozoa, which is one of the categories germs fit into. Protozoa are simple one-celled eukaryotic organisms that can make you sick, depending on the protozoan.
Other types of germs are bacteria, fungi, and viruses. Bacteria can be good for you and bad for you. Some fungi are good to eat, but some are poisionous as well. Viruses are almost universally known for their debilitating effects, they kill the cells they invade.