Germ Theory: Pasteur, Koch and Ehrlich

For centuries, no one knew what actually caused disease. Two wrong ideas dominated, and both had to be overturned before medicine could move forward.

The first was miasma theory — the belief that disease was spread by bad air or foul smells rising from rubbish, sewage and rotting matter. It seemed to fit, because disease was common in filthy, smelly places.

The second was spontaneous generation — the belief that microbes (tiny living things sometimes seen under early microscopes) were created by decaying matter, rather than being the cause of the decay. On this view, germs were a by-product of illness, not its source.

The key mistake: people saw microbes in rotting or diseased material and assumed decay produced the germs. Germ Theory reversed this: the germs cause the decay and the disease.

Understanding this reversal is essential. Every later breakthrough — vaccines, antiseptics, public-health reform — depended on people first accepting that invisible microbes, not bad air, made people ill.

Louis Pasteur was a French chemist, not a doctor. Asked by brewers to explain why beer, wine and milk went sour, he showed that this was caused by microbes in the air, not by the liquid itself.

To prove it, Pasteur designed his famous swan-necked flask experiment. He boiled broth to kill any microbes, then left it in flasks with long, curved necks. The curve trapped dust and microbes from the air, so the broth stayed fresh. When he tilted a flask so the broth touched the trapped microbes, it went off. This disproved spontaneous generation: the microbes came from the air, not from the broth.

In 1861 Pasteur published his Germ Theory of disease, arguing that microbes in the air caused decay and could cause disease. Later he applied the theory to prevention, developing vaccines against chicken cholera, anthrax (publicly tested in 1881) and, most famously, rabies in 1885.

Pasteur's contribution	Detail
Swan-necked flask experiment	Proved microbes come from the air, disproving spontaneous generation
Germ Theory (1861)	Established that microbes cause decay and disease
Vaccines	Developed vaccines for chicken cholera, anthrax and rabies (1885)

Pasteur proved that germs in general cause disease. The German doctor Robert Koch took the crucial next step: he identified the specific microbe responsible for each disease.

Koch developed methods that let scientists see and grow individual germs:

• He used agar jelly in flat dishes to grow pure colonies of a single bacterium.
• He used industrial dyes to stain bacteria, making them visible under the microscope.
• He photographed microbes, so his findings could be shared and checked.

Using these techniques, Koch isolated the bacteria that cause anthrax (1876), the microbe behind septicaemia (blood poisoning), the bacterium for tuberculosis (TB) in 1882, and the microbe for cholera in 1883. This proved that different diseases were caused by different, identifiable germs.

Pasteur and Koch became scientific rivals, France against Germany. Their competition, funded by their governments, drove both to work faster — a clear example of how science, individuals and government interacted to speed up progress.

Identifying germs was one thing; killing them inside the body without harming the patient was another. This was the achievement of Paul Ehrlich, a German scientist who had worked with Koch.

Ehrlich reasoned that if dyes could stain specific bacteria, a chemical might be found that would seek out and destroy a specific microbe while leaving the patient unharmed. He called such a chemical a magic bullet.

His team tested hundreds of arsenic-based compounds against the microbe that causes syphilis. In 1909 the 606th compound worked. Named Salvarsan 606, it was the first successful magic bullet and the beginning of chemotherapy — using chemicals to cure disease.

Scientist	Key breakthrough	Date	Significance
Pasteur	Germ Theory; vaccines	1861; rabies 1885	Proved microbes cause disease
Koch	Identified specific microbes (anthrax, TB)	1876; 1882	Linked each disease to its own germ
Ehrlich	Salvarsan 606 (magic bullet)	1909	First chemical cure; began chemotherapy

A second magic bullet, Prontosil, was found in the 1930s, but the search for magic bullets was slow. The real breakthrough in curing bacterial infections would come later, with penicillin.

Germ Theory was a French and German achievement, and it took time to change medicine in Britain — the focus the AQA specification requires. Ideas do not transform a country overnight.

At first, many British doctors were sceptical. Pasteur was a chemist, not a physician, and germs were invisible to most people. But over the following decades the theory reshaped British medicine and public life:

• Joseph Lister applied Germ Theory to surgery, using carbolic acid as an antiseptic and publishing his results in 1867.
• Others, such as the British scientist John Tyndall, publicly supported Pasteur and helped disprove spontaneous generation, making the theory harder to reject.
• Germ Theory gave a scientific reason for public-health reform: if germs in dirty water and waste caused disease, then clean water and sewers would save lives.

Germ Theory did not immediately produce cures. Its power was in understanding: once people knew what caused disease, they could work out how to prevent and treat it.

AQA's essay question asks you to weigh factors behind medical change. Germ Theory shows several factors working together.

Factor	How it drove Germ Theory forward
The individual	Pasteur, Koch and Ehrlich each made decisive personal breakthroughs
Science & technology	Better microscopes, dye-staining and agar plates made germs visible and testable
Government	France and Germany funded research, partly out of national rivalry
Communication	Printed papers and Koch's photographs let discoveries spread and be checked
Chance	Ehrlich's team stumbled on the effective compound only after hundreds of failures

The lesson for the essay is that no single factor explains the revolution. Pasteur was a brilliant individual, but he needed the technology of the microscope, the funding of government, and the communication of scientific journals for his ideas to spread and be built upon.

1. Confusing the cause of disease with its transmission or cure

Germ Theory explained what caused disease: microbes. It did not, on its own, explain how a disease spread through a city, nor did it immediately cure anyone. The examiner's report warns that students lose marks by muddling cause, transmission and treatment. Keep the three separate.

2. Getting spontaneous generation the wrong way round

Spontaneous generation claimed decay produced germs. Pasteur proved germs cause the decay. State the reversal clearly, or you misrepresent the whole breakthrough.

3. Muddling Pasteur and Koch

Pasteur proved germs cause disease and made vaccines; Koch identified the exact microbe for each disease. Do not credit Pasteur with isolating the TB bacterium (that was Koch, 1882), or Koch with Germ Theory itself (that was Pasteur, 1861).

4. Assuming Germ Theory changed British medicine instantly

Pasteur published in 1861, but the theory took decades to be accepted and applied in Britain. Higher-level answers show change over time, not an overnight transformation.

5. Ignoring the other factors in an essay

Naming individuals alone caps your mark. Strong answers explain how science and technology (microscopes, staining), government funding and communication combined with brilliant individuals to make the revolution possible.