Germ Theory: Pasteur, Koch & Disease
The Breakthrough: Germ Theory
For centuries, most people believed disease was caused by miasma — bad smells and foul air. Others clung to the ancient Theory of the Four Humours. Neither explanation was correct, but without scientific proof, these ideas persisted for hundreds of years. It took a French chemist working on a completely different problem to change everything.
In 1861, Louis Pasteur published his Germ Theory, proving that microorganisms caused decay. This was a turning point in the history of medicine — it meant disease was caused by specific germs, not by bad air or divine punishment.
Pasteur’s Discovery
Pasteur was not a doctor — he was a chemist. He was originally asked by the French wine industry to investigate why wine and beer were going sour. Through careful experiments using swan-necked flasks, Pasteur proved that it was microorganisms in the air that caused liquids to decay, not the air itself. His work on fermentation showed that living microbes, not the air itself, made wine and beer go sour. He published his findings in 1861.
Pasteur then extended his theory to argue that if germs caused decay, they must also cause disease in humans. This was revolutionary. However, Pasteur could not identify exactly which germs caused which specific diseases. He had proved the principle, but not the detail.
Germ Theory was a turning point because it shifted medical thinking from treating symptoms to understanding causes. Before 1861, doctors treated patients based on guesswork — bleeding, purging, and herbal remedies. After Germ Theory, scientists could begin to target the actual cause of disease. It opened the door to vaccinations, antiseptic surgery, and eventually antibiotics.
However, Germ Theory was not accepted immediately. Many doctors were resistant to new ideas, and it took years before its implications were fully applied in hospitals and public health.
Robert Koch: Identifying Specific Microbes
The German scientist Robert Koch built directly on Pasteur’s work. Where Pasteur proved that germs caused disease in general, Koch developed methods to identify which specific microbes caused which specific diseases.
Pasteur’s Germ Theory
Pasteur proves microorganisms cause decay, publishing his Germ Theory.
Koch Identifies Anthrax
Koch identifies the specific bacterium causing anthrax — the first time a disease was linked to a particular germ.
Koch Identifies Tuberculosis
Koch discovers the bacterium responsible for tuberculosis, one of the biggest killers of the 19th century.
Koch Identifies Cholera
Koch identifies the cholera bacillus, proving the waterborne nature of the disease.
Koch developed a method of staining bacteria with dyes so they could be seen more clearly under a microscope. He also grew bacteria in agar culture dishes, allowing him to isolate and study individual types of germ. His team identified the bacteria responsible for anthrax (1876), tuberculosis (1882), and cholera (1883).
Pasteur and Koch were fierce rivals, partly driven by national competition between France and Germany after the Franco-Prussian War (1870–71). This rivalry actually accelerated medical progress, as each scientist pushed to outdo the other in identifying germs and developing vaccines.
Once specific germs had been identified, scientists could develop targeted treatments and preventions. Pasteur himself developed vaccines for chicken cholera (1879) and rabies (1885). Koch’s identification of the tuberculosis bacterium eventually led to the BCG vaccine in the 20th century.
Germ Theory also influenced surgery. Joseph Lister used Pasteur’s ideas to develop antiseptic surgery in 1867, spraying carbolic acid during operations to kill germs. Infection rates and death rates from surgery dropped dramatically.
The discovery also supported the argument for improving public health. If germs in dirty water and sewage caused disease, then governments had a responsibility to provide clean water and sanitation — a key factor in the Public Health Act of 1875.
Factors Driving Change
The development of Germ Theory illustrates several key factors that the Edexcel specification asks you to consider. Science and technology were crucial — improved microscopes allowed Pasteur and Koch to observe microorganisms for the first time. Individuals mattered enormously — without Pasteur’s curiosity and Koch’s methodical approach, progress would have been slower. Attitudes in society also played a role: many doctors resisted Germ Theory because it challenged centuries of accepted medical practice, meaning change was gradual rather than instant.
The role of institutions should not be overlooked. Governments and universities funded research, and the rivalry between France and Germany created a competitive environment that pushed both nations to invest in medical science.
Limitations and Opposition
Despite its significance, Germ Theory faced considerable opposition. Many established doctors dismissed Pasteur’s findings because he was a chemist, not a trained physician. The medical profession was conservative, and changing long-held beliefs about miasma required more than a single experiment. Some critics argued that Pasteur had only shown germs caused decay in liquids, not disease in humans.
Even Koch’s more detailed work did not win universal acceptance immediately. His attempt to develop a cure for tuberculosis — a substance called tuberculin — failed publicly in 1890, temporarily damaging his reputation. Nevertheless, the broader principle of Germ Theory was gradually accepted as more and more bacteria were identified and linked to specific diseases throughout the 1880s and 1890s. By the early 20th century, the germ theory of disease was the foundation of modern medicine.