In life, Louis Pasteur was an historical French scientist and one of the founders of microbiology, that among other things, identified the microbes as the main responsible for diseases and infections. He was renowned for his discoveries of the principles of vaccination, microbial fermentation, and pasteurization. He (apparently) died in 1895. His apparent death to the mortal world, however, was nothing more than a ruse.
Years earlier, Pasteur had been contacted by a vampire of unknown origins named Georges. Impressed with Pasteur's advances in the field of biology and studies of diseases, he approached the French doctor, seeking help to find a cure for his condition. Although Pasteur's research failed to produce results at the time, the scientist did manage to store the vampiric blood, and succeeded in developing a way to turn humans into vampires through inoculation. It was using this method that Pasteur, already feeling the approach of death, managed to gain extra time to work on his research.
When it became apparent that Pasteur was soon to die, Georges disappeared, but the scientist continued his research and injected himself with the serum he and Georges had developed. That night, he died only to live again, now as a vampire himself. Assuming several fake names during in the following years, the doctor finally settled in Denver, Colorado, under the guise of Jacob Prestor. There, he managed to develop a serum that was indeed capable of reverting a vampire to the mortal condition, under an array of specific circumstances and not without much danger to the serum's subject.
This miraculous serum, however, caught the attention of the Nosferatu methuselah, Thaddeus, who believed that Pasteur and his findings were a threat to the Kindred, and manipulated the Prince of Denver, Edward Williams, into ordering Pasteur's Final Death. After a desperate plot to create a vampire circle capable of protecting him failed, Pasteur finally met his Final Death at the hands of his enemies.
For the historical figure, see Louis Pasteur.