Germ theory, or the pathogenic theory of medicine, postulates that micro-organisms are the cause of many diseases. Although originally controversial, Louis Pasteur in the 19th century proved beyond doubt that harmful bacteria within the body are the cause of many diseases. His research is now regarded as one of the central foundations of modern medicine, especially in the areas of immunology and modern hygienic practices in both hospitals and wider society.
According to J.I. Rodale (1956), Pasteur's germ theory can be easily summarized simply as "Germs live in the air, every once in a while get into a human body, multiply and cause illness. Nothing to it at all. All you have to do is kill the germs and disease is licked." Germ theory became popular in the 19th century as it provided man with a readymade and easily understood scapegoat for disease, being those nasty little organisms that go around attacking man and his immune system. It also fit nicely into the prevalent philosophy of the time of mechanistic theories of the universe. It should be noted at this point that Pasteur is not the father of germ theory but is arguably the main developer and researcher of it. It is generally thought that germ theory was originally developed by Girolamo Fracastoro, Agostino Bassi, and Friedrich Henle among others.
Ironically, germ theory, which is now take as a global truth was not even developed by a doctor, but instead by the famous French chemist Louis Pasteur, who discovered the harmful effects of germs on the body during work on fermentation of beer, diseases in wines and silkworms. He was spurred to develop a medical application for his research by the death of his eldest daughter who died from typhoid fever
Pasteur's and later Robert Koch's work on germ theory eventually led to the creation of antibiotics which were used to kill and destroy the harmful bacteria residing in the body. This cure was seen at the time as a cellular manifestation of Darwinism in which all creatures had to fight to gain supremacy and survive to the detriment and destruction of their competitor. The introduction of antibiotics into the body was seen as akin to a new top level predator being introduced to an environment which would kill the harmful bacteria, but not themselves harm the host.
The development of the microscope in the early 1800's enabled Pasteur to be able to see microbes at work in the production of alcohol and this led to his looking at other bacteria that were proving harmful to animals and humans. Without the development of the microscope, scientists like Pasteur would not have been able to see the bacteria of diphtheria, measles, tetanus, typhoid and cholera, the killer diseases of the era.
Pasteur swore to Napoleon that he would come up with a cure for microbial disease and after isolating two different microbes that caused disease in silkworms ended up creating a screening mechanism for silkworm eggs that saved the French silkworm industry and was another milestone in his groundbreaking work.
The final proof that germ theory was valid came in 1876 when he proved that the bacteria anthrax-bacillus was the cause of charbon (anthrax), a deadly animal disease, rather than through the transmission of blood. With the success of this experiment he proved the validity of germ theory, although it took another generation for doctors and scientists to be fully convinced.
Pasteur's work on germ theory also led to the accidental discovery of immunization when he injected a ruined test sample of chicken cholera into a diseased chicken. The chicken recovered and the field of immunology was born. Pasteur's early work on immunization also led to him saving two boys from the fatal disease rabies.
The field of germ theory was developed as a counter to the cellular theory postulated by Bechamp amongst others. Bechamp believed that cells in the body called microzymas could develop into harmful bacteria within the body which would then cause diseases through a process of fermentation. Germ theory sees the cause of disease as the other way round with bacteria in the body infecting the cells.
Germ theory also contributed to the rise of basic sanitation seen in England and other industrialized countries in the latter half of the 19th century. Governments realized that harmful disease could be controlled by basic sanitation and hygiene, and began to construct modern sewerage systems which led to a large increase in the life expectancy of the population, especially among the lower classes.