Louis Pasteur devised the first vaccines for humans, thereby saving millions of lives by long term prevention of disease. He also challenged the myth on spontaneous generation, thereby setting the stage for modern biology and biochemistry.
While studying alcohol fermentation, he was confronted with a problem on improper fermentation. Instead of the by-product alcohol, lactic acid was produced.
In the course of his research Louis Pasteur subjected the mixture to high temperature; killing in effect the microorganism, thereby sterilizing (pasteurizing) it before introducing pure cultures of microbes and yeast from where a predictable fermentation was achieved.
Destroying the myth of spontaneous generation
Pasteur debunked the theory that beetles, maggots, eels and microbes could arise simultaneously from decaying matter. By a simple experiment he was able to prove that under no circumstance can microscopic beings be born into the world without germs, without parents similar to themselves. Thus he single handedly extinguished the fire on 'spontaneous generation'.
The Germ Theory
The crowning achievement of his career was his "germ theory of disease". It brought to the open how specific microorganisms can bring about important diseases such as cholera, diphtheria, scarlet fever, childbirth fever, syphilis and smallpox. Though the medical community was late to admit, in the end they did follow Pasteur's lead.
Cows, sheep, chicken...and barking dogs
In the course of his lifetime Louis Pasteur was able to develop vaccines for anthrax and chicken cholera, helping the dairy and poultry industry rise on its feet.
Louis Pasteur's final gift to humanity involved the development of the anti-rabies vaccine, which proved to be very successful.