Robert Koch
Does every disease have its own microbe?
That time, that place
He began as a country doctor. It started with a microscope his wife gave him for his birthday, and the anthrax spreading through local livestock.
Why this question
Pasteur had argued that microbes cause disease, but there was no way to prove which microbe causes which disease.
Finding a germ in a sick person is not enough. It may be the cause, or it may have moved into a body already weakened by the illness. Being present and being the cause are not the same.
What was found
Koch built a procedure for proof.
The germ must be found in every case of the disease. It must be grown in pure culture outside the body. The cultured germ, introduced into a healthy subject, must produce the same disease. And the same germ must be recovered from that subject. Only after all four can you call it the cause.
Doing this required technique. He developed solid culture media in place of broth, so that a colony grown from a single organism could be isolated. The suggestion to use agar as the solidifying agent came from an assistant's wife. He stained germs to make them visible and photographed them through the microscope so that others could check.
In 1882 he identified the tuberculosis bacillus. One European in seven was then dying of it.
A germ found in a sick body was taken to be the cause
Reproducing the same disease with a pure culture of the isolated germ
The four steps remain the basic frame for establishing infectious causation, though exceptions such as asymptomatic carriers are now known