Germ Theory: Pasteur and Koch

The development of germ theory in the second half of the 19th century was one of the most important turning points in the history of medicine. For the first time, scientists proved that diseases were caused by microorganisms (germs), overturning centuries of belief in miasma and the humours.

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The Problem: What Causes Disease?

Before germ theory, the dominant explanations for disease were:

Theory	Explanation
Miasma	Disease was caused by "bad air" arising from rotting organic matter
Spontaneous generation	Germs appeared as a result of disease, not as a cause
The Four Humours	Disease was caused by an imbalance of bodily fluids
God's punishment	Illness was divine retribution for sinful behaviour

Key Term: Spontaneous generation --- the widely held belief that microorganisms arose spontaneously from decaying matter, rather than being the cause of decay. Pasteur disproved this theory.

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Louis Pasteur (1822--1895)

Pasteur was a French chemist who made the critical breakthrough in understanding the role of germs in causing disease.

Key Discoveries

Date	Discovery	Significance
1857	Showed that microorganisms caused the souring of wine and beer	Proved that germs were responsible for fermentation
1861	Germ Theory published	Proved that germs cause decay (not the other way around); disproved spontaneous generation
1867	Applied germ theory to human disease	Suggested that specific germs caused specific diseases, though he did not identify which germs caused which diseases
1879	Developed a vaccine for chicken cholera	Discovered that weakened germs could provide immunity (the principle behind vaccination)
1881	Developed a vaccine for anthrax	Further proved the principle of using weakened germs
1885	Developed a vaccine for rabies	Saved the life of a boy bitten by a rabid dog

Pasteur's Swan-Neck Flask Experiment (1861)

Pasteur designed a brilliant experiment to disprove spontaneous generation:

1. He boiled broth in a flask with a long, curved "swan neck."
2. Air could enter the flask, but germs became trapped in the curve of the neck.
3. The broth remained clear and free from decay --- proving that germs in the air (not the air itself) caused decay.
4. When the neck was broken off, germs entered and the broth decayed rapidly.

Exam Tip: Pasteur's germ theory is the single most important breakthrough in the history of medicine according to many historians. Be prepared to explain why --- it provided the scientific basis for understanding disease, vaccination, antiseptic surgery, and public health reform.

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Robert Koch (1843--1910)

Koch was a German doctor who built on Pasteur's work by identifying the specific germs that caused individual diseases.

Key Discoveries

Date	Discovery	Significance
1876	Identified the bacterium causing anthrax	First time a specific germ was linked to a specific disease
1882	Identified the bacterium causing tuberculosis (TB)	TB was the biggest killer of the 19th century; this was a major breakthrough
1883	Identified the bacterium causing cholera	Confirmed John Snow's waterborne theory from 1854

Koch's Methods

Koch developed new laboratory techniques that transformed bacteriology.

Method	Detail
Staining bacteria	Used chemical dyes to make bacteria visible under the microscope
Growing bacteria in colonies	Developed methods to grow pure cultures of bacteria using agar jelly and Petri dishes
Koch's Postulates	A set of rules for proving that a specific germ causes a specific disease

Key Term: Koch's Postulates --- a set of criteria for proving the link between a microorganism and a disease: (1) the microbe must be found in all cases of the disease; (2) it must be isolated and grown in a pure culture; (3) it must cause the disease when introduced into a healthy organism; (4) it must be re-isolated from the newly diseased organism.

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Pasteur vs Koch: The Rivalry

The relationship between Pasteur and Koch was shaped by national rivalry (France vs Germany) as well as scientific competition.

Pasteur	Koch
French	German
Chemist by training	Doctor by training
Developed the overall theory (germ theory)	Identified specific disease-causing germs
Created vaccines (chicken cholera, anthrax, rabies)	Developed laboratory techniques for studying bacteria

Exam Tip: The Franco-Prussian War (1870--71) intensified the rivalry between Pasteur and Koch. Each wanted to prove their nation's scientific superiority. This competition actually accelerated medical progress, which is a useful point to make in exam answers about factors that drive change.

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The Impact of Germ Theory

Area	Impact
Understanding disease	For the first time, doctors knew what actually caused disease
Vaccination	Pasteur's work led to new vaccines for chicken cholera, anthrax, and rabies
Antiseptic surgery	Lister used Pasteur's ideas to develop carbolic acid spray (1867)
Public health	Provided scientific justification for clean water, sewage systems, and housing reforms
Limitation	It took time for germ theory to be accepted; many doctors continued to believe in miasma

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Key Dates

Date	Event
1861	Pasteur publishes germ theory
1867	Lister uses carbolic acid as an antiseptic
1876	Koch identifies the anthrax bacterium
1882	Koch identifies the TB bacterium
1885	Pasteur develops the rabies vaccine

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Extended Worked Paragraph: Level 5 Model on a Q4 "Main Factor" Question

Question: "Has science been the main factor in the understanding of disease in the 19th century?" (16 marks + 4 SPaG)