> By the late 1870s, scientists learned about bacteria. Bacteria had been discovered in the 17th Century, the moment the microscope was invented. I think by R

> By the late 1870s, scientists learned about bacteria. Bacteria had been discovered in the 17th Century, the moment the microscope was invented. I think by Robert Hooke, but I could have misremembered that. What was new in the 1870s was the germ theory of disease.