World-renowned French chemist and biologist, who founded the science of microbiology, proved the germ theory of disease, invented the process of pasteurization, and developed vaccines for several diseases, including rabies.   

Pasteur was born in Dôle on December 27, 1822, the son of a tanner, and grew up in the small town of Arbois. In 1847 he earned a doctorate at the École Normale in Paris, with a focus on both physics and chemistry. Becoming an assistant to one of his teachers, he began research that led to a significant discovery. He found that a beam of polarized light was rotated to either the right or the left as it passed through a pure solution of naturally produced organic nutrients, whereas when polarized light was passed through a solution of artificially synthesized organic nutrients, no rotation took place.  
If, however, bacteria or other microorganisms were placed in the latter solution, after a while it would also rotate light to the right or left.   
Pasteur concluded that organic molecules can exist in one of two forms, called isomers (that is, having the same structure and differing only in mirror images of each other), which he referred to as "left-handed" and "right-handed" forms. When chemists synthesize an organic compound, both of these forms are produced in equal proportions, canceling each other's optical effects. Living systems, however, which have a high degree of chemical specificity, can discriminate between the two forms, metabolizing one and leaving the other untouched and free to rotate light.   
Work on Fermentation  
 
After spending several years of research and teaching at Dijon and Strasbourg, Pasteur moved in 1854 to the University of Lille, where he was named professor of chemistry and dean of the faculty of sciences. This faculty had been set up partly to serve as a means of applying science to the practical problems of the industries of the region, especially the manufacture of alcoholic beverages. Pasteur immediately devoted himself to research on the process of fermentation. Although his belief that yeast plays some kind of role in this process was not original, he was able to demonstrate, from his earlier work on chemical specificity, that the desired production of alcohol in fermentation is indeed due to yeast and that the undesired production of substances (such as lactic acid or acetic acid) that make wine sour is due to the presence of additional organisms such as bacteria. The souring of wine and beer had been a major economic problem in France; Pasteur contributed to solving the problem by showing that bacteria can be eliminated by heating the starting sugar solutions to a high temperature ( 55 - 60^oC for a few minutes). 
Pasteur extended these studies to such other problems as the souring of milk, and he proposed a similar solution: heating the milk to a high temperature and pressure before bottling. This process is now called pasteurization.  
  
Disproof of Spontaneous Generation   
Fully aware of the presence of microorganisms in nature, Pasteur undertook several experiments designed to address the question of where these "germs" came from. Were they spontaneously produced in substances themselves, or were they introduced into substances from the environment? Pasteur concluded that the latter was always the case. His findings resulted in a fierce debate with the French biologist Félix Pouchet-and later with the noted English bacteriologist Henry Bastion-who maintained that under appropriate conditions instances of spontaneous generation could be found. These debates, which lasted well into the 1870s, although a commission of the Académie des Sciences officially accepted Pasteur's results in 1864, gave great impetus to improving experimental techniques in microbiology.  Silkworm Studies   
In 1865, Pasteur was summoned from Paris, where he had become administrator and director of scientific studies at the École Normale, to come to the aid of the silk industry in southern France. The country's enormous production of silk had suddenly been curtailed because a disease of silkworms, known as pébrine, had reached epidemic proportions. Suspecting that certain microscopic objects found in the diseased silkworms (and in the moths and their eggs) were disease-producing organisms, Pasteur experimented with controlled breeding and proved that pébrine was not only contagious but also hereditary. He concluded that only in diseased and living eggs was the cause of the disease maintained; therefore, selection of disease-free eggs was the solution. By adopting this method of selection, the silk industry was saved from disaster.  Germ Theory of Disease   
Pasteur's work on fermentation and spontaneous generation had considerable implications for medicine, because he believed that the origin and development of disease are analogous to the origin and process of fermentation. That is, disease arises from germs attacking the body from outside, just as unwanted microorganisms invade milk and cause fermentation.  
This concept, called the germ theory of disease, was strongly debated by physicians and scientists around the world. One of the main arguments against it was the contention that the role germs played during the course of disease was secondary and unimportant; the notion that tiny organisms could kill vastly larger ones seemed ridiculous to many people. Pasteur's studies convinced him that he was right, however, and in the course of his career he extended the germ theory to explain the causes of many diseases.  Anthrax Research   
 
Pasteur also determined the natural history of anthrax, a fatal disease of cattle. He proved that anthrax is caused by a particular bacillus and suggested that animals could be given anthrax in a mild form by vaccinating them with attenuated (weakened) bacilli, thus providing immunity from potentially fatal attacks. In order to prove his theory, Pasteur began by inoculating 25 sheep; a few days later he inoculated these and 25 more sheep with an especially strong inoculant, and he left 10 sheep untreated. He predicted that the second 25 sheep would all perish and concluded the experiment dramatically by showing, to a skeptical crowd, the carcasses of the 25 sheep lying side by side.  Rabies Vaccine   
Pasteur spent the rest of his life working on the causes of various diseases-including septicemia, cholera, diphtheria, fowl cholera, tuberculosis, and smallpox-and their prevention by means of vaccination. He is best known for his investigations concerning the prevention of rabies, otherwise known in humans as hydrophobia. After experimenting with the saliva of animals suffering from this disease, Pasteur concluded that the disease rests in the nerve centers of the body; when an extract from the spinal column of a rabid dog was injected into the bodies of healthy animals, symptoms of rabies were produced. By studying the tissues of infected animals, particularly rabbits, Pasteur was able to develop an attenuated form of the virus that could be used for inoculation.   
In 1885, a young boy and his mother arrived at Pasteur's laboratory; the boy had been bitten badly by a rabid dog, and Pasteur was urged to treat him with his new method. At the end of the treatment, which lasted ten days, the boy was being inoculated with the most potent rabies virus known; he recovered and remained healthy. Since that time, thousands of people have been saved from rabies by this treatment.  
Pasteur's research on rabies resulted, in 1888, in the founding of a special institute in Paris for the treatment of the disease. This became known as the Institut Pasteur, and it was directed by Pasteur himself until he died. (The institute still flourishes and is one of the most important centers in the world for the study of infectious diseases and other subjects related to microorganisms, including molecular genetics.) By the time of his death in Saint-Cloud on September 28, 1895, Pasteur had long since become a national hero and had been honored in many ways. He was given a state funeral at the Cathedral of Nôtre Dame, and his body was placed in a permanent crypt in his institute.