Before the 1800s, at least in Western medicine, the causes of disease were not so obvious. They ranged from demons, filth and miasma (like bad smells caused by rotting flesh or excrement).
During that time there were a number of diseases plaguing people. Chief among them was cholera, a water-borne disease that causes severe diarrhoea, vomiting and dehydration. While some people thought the disease was caused by smelly things or bad spirits, scientists like Jacob Henle and Louis Pasteur had other ideas.
Henle developed the notion that disease was a chemical process caused by microscopic organisms that infiltrate the body. He called these micro-chemicals and organisms germs and later developed Germ theory. Louis Pasteur, a French biologist, proved the Germ theory in a radical experiment a few years later.
In 1880, Pasteur cultivated cholera in a lab and infected healthy chickens. They died as he expected. A few days later, he took what was left of his cholera cocktail and inoculated more chickens but those ones lived. He postulated that the cholera had weakened in the preceding days, which explains why the Chickens survived. Then he made a fresh, more potent mix to infect the chickens again. However, they lived.
He tried the same experiment with other diseases – to weaken the disease first then infect the subject. Then a few days later, infect it again with a stronger brew. Again, he found that the animals (mostly chickens and birds) became immune and did not get affected by the second inoculation. Today, of course, we have vaccines which work exactly like that.
As it turns out, what does not kill you indeed makes you stronger.
