There was a time not so very long ago in human history when we believed that miasmas (bad air) caused illness. Doctors didn’t even wash their hands between seeing patients because how could gentlemen be responsible for spreading illness?

There were no microscopes, so no one could look and see bacteria doing battle with our own immune system. People could only know what they observed and what they observed was that the areas where disease was most prevalent were also very smelly from the dead and dying people.

We know now, of course, that the smell so common in areas where poor people lived wasn’t the cause of their illness, but another product of the things making them sick. Bodies and human waste (lots of poo) left in the streets, rotting food, animals and humans living in close proximity, sewage in the water. Germ Theory tells us that these things become the breeding grounds of virus, bacteria, fungi, and other tiny critters which make us sick. By eliminating those risks from our environment, we eliminate a lot of the pathways those germs take to making us ill.