Pasteur inoculating a man with the rabies virus

The history of germ theory

Jemima Hodkinson explores germ theory and two of the scientists behind it: Louis Pasteur and Robert Koch

Today, we understand that infectious diseases like flu, chickenpox and pneumonia are caused by microscopic organisms – bacteria and viruses. Without this knowledge, we might have never developed ways to treat and prevent such infections. However, this understanding – known as the ‘germ theory’ of disease – was a remarkably recent discovery.

People have created theories to explain human disease for millennia: the Greek physician Hippocrates, born in 460 BCE, thought that ‘bad air’ from swampy areas was to blame. In the 19th century, improvements in microscope technology enabled a generation of microbiologists to investigate further the world of previously unseen disease-causing organisms.

Many of these scientists carried out research that contributed towards the formation of the germ theory. However, scientific proof of the theory was the achievement of two European scientists: Louis Pasteur, a Frenchman, and Robert Koch, who was German.

The birth of pasteurisation

Pasteur was a chemist: his early research focused on the study of crystals. But when he took up the post of head of the Science Faculty in Lille in 1854, he was inundated with demands from the local wine industry for him to research the science of fermentation. For the first time, Pasteur discovered that the process was caused by a living organism, which he called ‘ferment’. This work was a turning point in the young chemist’s career – he now began to apply his rigorous experimental methods to biological questions.

By discovering that fermentation was caused by living organisms, Pasteur had raised many issues: what organisms did ‘ferment’ consist of, and where did these organisms come from? Many other scientists at the time believed that such microorganisms appeared out of thin air – the so-called ‘spontaneous generation’ theory.

The alternative opinion was that these microorganisms originated from other similar microscopic beings. Pasteur set out to conduct a series of experiments that would conclusively resolve the debate.

Essential to these tests was an unusual glass flask with a long, thin, bent tube attached to the neck – Pasteur called it a swan-necked flask. Using these flasks, he boiled liquids (therefore killing all of the microorganisms inside) and then left them to cool. The design of the flask allowed the boiled liquid to be in contact with the air, while preventing any dust or dirt from entering.

Pasteur tested many different liquids in this way, including those which usually fermented very easily. He found that none of them fermented after being boiled. He concluded that the processes of fermentation and decay were caused by microorganisms present in the air, and that these microorganisms could be killed by heating.

Pasteur applied this new understanding to the local industries in Lille: most famously, he developed a simple way to prevent wine being contaminated by unwanted microorganisms. This involved heating the wine to 50–60°C, a technique we now know as pasteurisation, after him. It is still used in the production of many foodstuffs.

However, the most dramatic consequences of Pasteur’s discovery were in the biology of disease. Using the experimental techniques he had begun to develop in his earlier work, Pasteur went on to discover several species of bacteria. He also developed ways of making bacteria and viruses less dangerous so that they could be used for vaccination – the technique for preventing disease discovered earlier by English doctor Edward Jenner. (See History of vaccination for more.)

Proving a point

The huge task of matching specific microorganisms to the disease they cause now lay ahead. About ten years after Pasteur’s famous fermentation experiments, in a small self-built laboratory in the countryside near Berlin, the German microbiologist Robert Koch was working to establish a new method for approaching this task.

The first disease he chose to study was anthrax, as it was common among farm animals in the area where he lived. The anthrax bacterium had already been discovered, but nobody had proved that this particular bacterium caused the disease seen in animals.

Koch collected anthrax bacteria from farm animals that had died of the disease and used them to infect healthy mice. He also carried out a control experiment, using exactly the same method but substituting the anthrax bacilli with blood from healthy farm animals. The anthrax-infected mice developed the disease and died, but the control mice remained healthy: Koch now had clear evidence that the anthrax bacteria had caused the disease. He then grew pure samples of anthrax bacteria and showed that these could also cause the disease.

As soon as the right method was found, discoveries came as easily as ripe apples from a tree.

These results were published and quickly recognised as having far-reaching consequences for microbiology. Building on this work, he developed new ways to grow pure samples of bacteria and stain them so they were visible under a microscope.

However, Koch is best known today for devising a universal method for testing whether a specific bacterium causes a particular disease, known as ‘Koch’s postulates’. Using this method, he was able to discover the bacterium that causes tuberculosis – a major killer of the 19th century. The Nobel Prize in Physiology or Medicine was awarded to Koch in 1905 in recognition of his contribution to bacteriology and the understanding of disease.

By 1900 the discoveries of Pasteur and Koch, and the work of their fellow scientists, had led to the identification of 21 disease-causing microorganisms in just over two decades. As Koch himself said, “As soon as the right method was found, discoveries came as easily as ripe apples from a tree.”

Lead image:

Pasteur inoculating a man with the rabies virus.

Wellcome Library, London CC BY


Questions for discussion

  • What are Koch’s postulates? Try searching the term online.
  • Joseph Lister was a surgeon during the 19th century. He also made important contributions to the understanding of disease. Search online to find out about his work. What experiments did he do, and how did they support the germ theory of disease?
  • Before germ theory became widely accepted, people had other theories to explain the causes of disease. Research two of these and write a paragraph about each. The links above and below might be useful (‘From miasmas to germs’ in particular).

Further reading

About this resource

This resource was first published in ‘Immune System’ in January 2015.

Microbiology, Immunology, History, Health, infection and disease
Immune System
Education levels:
14–16, 16–19, Continuing professional development