Officially, the Germ Theory Of Disease is the scientific theory that certain diseases are caused by microorganisms, also known as pathogens which are too small to be seen without a microscope, and they can invade humans, other animals, and other living hosts. Their growth and reproduction within their hosts can cause disease.
It was first proposed by Italian physician Girolamo Fracastoro in the 16th century, but it was not widely accepted until the 19th century.
In the 1850s, Hungarian physician Ignaz Semmelweis showed that handwashing could prevent the apparent spread of Puerperal Fever, a deadly infection that often killed women after childbirth.
In the 1870s, German scientist Robert Koch developed a set of four postulates that can be used to prove that a particular microorganism causes a particular disease.
The microbe must be found in all individuals with the disease, but not in healthy individuals.
The microbe must be isolated from a diseased individual and grown in pure culture.
When the microbe from the pure culture is inoculated into a healthy individual, the individual must develop the disease.
The microbe must be re-isolated from the inoculated individual and shown to be the same microbe that was originally isolated.
Which is where things get blurry.
No virus, for example, has met the four postulates.
And, as it turns out, neither has any bacterium.
For example, the Rosenau Experiment, conducted during the 1918-1919 influenza pandemic, was a series of epidemiological studies led by Dr. Milton Rosenau at Harvard University, funded by a $50,000 donation. The experiments involved 100 navy volunteers with no history of influenza. They were exposed to various strains of Pfeiffer bacillus, mixtures of other organisms isolated from influenza patients, and blood from influenza patients. Some volunteers were even taken into an influenza ward and exposed to influenza patients directly.
Despite these exposures, none of the volunteers developed influenza, leading Rosenau to conclude that the understanding of the alleged disease’s transmission was not as clear as previously thought.
Andrew Kaufman broke it all apart in simple language.
A beginner’s guide to Germ Theory, with Andrew Kaufman: https://jermwarfare.com/conversations/beginners-guide-andrew-kaufman