Not that long ago, we didn’t understand why we got sick. There was no internet, and doctors were basically guessing.

But then, in the 19^th century, a few scientists figured it out: germs.

One of those scientists was Louis Pasteur. His name might ring a bell because he developed pasteurization, a process that kills off germs in things like milk.

Louis Pasteur was an accomplished French chemist when he started to investigate the causes of disease.

He developed some of the earliest vaccines, including one that virtually saved Europe’s chicken industry. His microbiology research also revived France’s silk production, and his work on pasteurization saved the French beer and wine industries.

Pasteur had become a science rockstar.

One of the key moments in persuading the world that germs caused disease occurred on a French farm in 1881.

Pasteur got into an argument with a veterinarian who absolutely refused to accept that germs might cause anthrax. Pasteur basically challenged him to a scientific duel.

In 1881, he went to a French village, had 50 sheep, perfectly healthy, set aside. 25 of them were given what Pasteur claimed to be an anthrax vaccine produced by weakening the bacteria which he claimed caused anthrax. The other 25 didn't get the injection. Not long after, all 50 sheep were injected with a particularly virulent strain of what Pasteur believed to be the anthrax bacteria.

This was huge, especially because press from around the world came to cover the results. And the reveal was pretty remarkable.

The 25 sheep that had had Pasteur's experimental vaccine were healthy, the others were dying really quite miserable deaths, clearly of anthrax.

This was splashed on the front pages of newspapers across the world. Pasteur was a master of PR, but it was necessary for people to fundamentally alter the way they thought about infectious disease to realize that microorganisms were indeed the cause.

One really important outcome of the germ revolution was the development of more vaccines.

Now, there had been smallpox vaccines. They'd been available for well over a century, but that was it. But as a result of the work of people like Louis Pasteur others, you see vaccines introduced for anthrax, for cholera and for typhoid.

One of the most important public health innovations was the introduction of pasteurization in reference to Louis Pasteur. The idea was that you would heat up a substance like milk, kill off the bacteria, and then it could be sold. This was particularly important for milk because of course infants were so heavily reliant on it and it dramatically reduced the number of children who were dying from gastrointestinal illness, one of the great killers of the 19th century.

In fact, in the 19^th century, 1 out of every 4 infants passed away before their first birthday due to things like infectious diseases. After the rise of germ theory and processes like pasteurization, the number of infant mortalities dropped – in some places – as much as 50%.

Just over a hundred years since that day on the French sheep farm, pasteurization became law in the United States. And in the decades in between, more vaccines were developed, including Louis Pasteur’s vaccine for rabies.

Medicine had been ineffective for virtually the entirety of human history. Now they were going down the right path… curative medicine came of age largely because doctors knew what actually causes disease.