Not until the 1880s were the bacteria that caused typhoid fever and a number of other diseases first isolated and identified with the aid of powerful microscopes. At that point, large numbers of physicians began to accept the idea that these bacteria were the cause of disease--the so-called "contagionist" point of view.

"Anti-contagionists" cited the clarity of water, witnessed by the naked eye after it had undergone gravity settling, as testimony of its purity. But their point of view became more and more marginalized as evidence mounted for spread of disease by microscopic germs.