The arrival of germ theory in the mid-1800s

From The Illustrious Dead by Stephan Talty.  Page 169.

The great difference between the French doctors struggling to come to grips with typhus at the hospital at Borodino and later researchers—or ourselves, watching them struggle blindly as thousands died—was the arrival of germ theory in the mid-1800s. In 1835, Agostino Bassi traced an epidemic killing Italian silkworms to infectious spores he observed under a microscope and became the first to formulate the idea of living, contagious agents as the cause of disease. John Snow’s tracking of the source of the 1854 London cholera epidemic, ending with his identification of the famous Broad Street Pump as the source of the contaminated water, advanced the cause of contagion, and Louis Pasteur proved in the early 1860s that spontaneous generation was superstition. But it was Robert Koch in 1876 who connected a pathogen—in this case, Bacillus anthracis—and a disease, anthrax.

As the medical historian Charles-Edward Amory Winslow has pointed out, by 1812 the key elements for a germ theory of disease had been in place for almost two hundred years. Antoine Philips van Leeuwenhoek had used primitive microscopes to identify bacteria in the human mouth. Athanasius Kircher had originated the idea of living organisms capable of transporting disease. Francisco Redi had demonstrated that piles of rags and other inanimate objects didn’t give rise to living things (there was no way, for example, that rats and insects could materialize from decomposing matter). As Winslow writes, “If an openminded and imaginative observer had put the work of these three pioneers together, the germ theory of disease could have been developed in the seventeenth century instead of the nineteenth."