Louis Pasteur was already a Professor of Chemistry when he was asked to investigate a problem facing certain wine producers in France. Some of it was turning into vinegar!
As a scientist he was meticulous in his methods and concluded that micro-organisms in the air were the cause. This was a revolutionary claim, because up until then micro-organisms were believed to result from putrefaction and decay. No one thought they caused it.
Experts of the day challenged Pasteur's idea, as it undermined their reputations and threatened to overturn the accepted doctrines. Undaunted, Pasteur took on his critics. In a series of public lectures he used two swan necked flasks to prove that microbes in the air did indeed cause liquids to 'go bad'. Both flasks contained broth which had been boiled to kill any microbes within it. One swan necked flask was left alone. The other was shaken vigorously to agitate the air in the neck. Result...the broth in the shaken flask began to go moldy after a few days, broth in the unshaken flask kept wholesome for months. Thus he not only discovered a key truth of great importance but devised and publicized a proof which his critics could not rebut.
Pasteur went on to consider whether microbes might cause disease and around 1864 had developed the Germ Theory. Again, this challenged the orthodoxy of his day, which was that germs resulted from disease (the Theory of Spontaneous Generation of Disease). However, a stroke interrupted his work before he could prove this hypothesis.
Following on from this, Robert Koch in Germany, having read Pasteur's work, began to investigate whether microbes caused disease. It was he who went on to prove Pasteur's hypothesis and to develop a method to isolate specific germs causing specific diseases, but Pasteur provided the initial inspiration.
After several years, Pasteur went back to work. Franco-German rivalry led to a race to develop ways to combat diseases and it was a research team under Pasteur's leadership which stumbled upon vaccination. Up to about 1870 only smallpox could be combated by vaccination, using cowpox vaccine. This had been shown by the English doctor Edward Jenner in 1798, but no one understood how it worked.
Whilst experimenting on chickens, injecting them with a disease called 'chicken cholera' which was invariably fatal, an accident occurred. Without anyone realizing, a batch of germs which had died was injected into some of the chickens. They did not die. Intrigued, the scientist, Dr Chamberland, drew Pasteur's attention to this. They were given fresh injections but still did not die.
Pasteur had the inspiration to examine the two batches of germs and realized that somehow the dead germs, given first, had protected the chickens from the second batch of live germs.
Following this up, he as able to weaken germs by drying them and use them to protect against disease. Again confronted by skeptics, he held a public experiment involving two batches of sheep to prove that his anthrax vaccine worked.
Although unable to identify the germ which caused rabies, he was able to isolate it in the spinal cord of infected animals. He ground up infected spinal cords to make a graded series of vaccines and famously tested it on a young boy who had been savaged by a rabid dog. The boy lived.
Arguably the single most important person in the development of modern medicine, Louis Pasteur stands as a giant who deserves to be remembered for much more than 'pasteurization'.