There were many factors influencing the development of vaccines in the later part of the 19th century.
Edward Jenner was the first man to begin the idea of vaccinations when he developed a vaccine against smallpox in 1798. Jenner's work established a new practise. The idea of infecting patients with a milder form of a disease had not been done before. Jenner began this practise, and injected a patient of his, James Phipps, with cowpox, which was thought to be a milder version of smallpox. He then injected Phipps with smallpox, but Phipps did not catch the disease. Jenner published a book in 1798 detailing his investigation, although it did not offer any explanation as to how the vaccination worked. This helped scientists such as Pasteur and Koch with their own work and experiments. .
Technology also had a big part to play in the development of new vaccines. During the 1800s, microscopes were built which were stronger that the ones used before. These helped scientists to observe how and why food decayed. As a result of this, a French scientist called Louis Pasteur began to develop the idea that decaying matter did not grow out of infected foods; that, instead, it was travelling through the air and thus spreading disease. .
Pasteur was able to prove his theory when he was asked to help a local brewer because his fermentation process to make alcohol was going wrong. After a series of careful experiments, Pasteur proved that microbes floated about in the air, landing on different substance, and so causing decay. He also experimented the effect of boiling food, placing it in a vacuum, and exposing it to air, and then recording the results he obtained. .
Pasteur's idea of the new germ theory' helped others, for example Robert Koch, to take the final step in identifying germs which caused certain diseases. Pasteur himself was not a doctor, and did not prove that germs caused diseases in humans. When an epidemic of cholera hit France in 1865, Pasteur tried hard to identify the germ causing the disease, but it was Koch who succeeded in doing this.