Step 1: The discovery of micro-organisms
In the late 1600s a Dutchman called Anthony van Leeuwenhoek made some of the earliest microscopes. They had only one lens, and the image was distorted and fuzzy. Almost everything he looked at contained tiny organisms which he called animalcules. He even found them in the waste material he scraped from between his own teeth. He described his findings in a series of 200 papers to the Royal Society in London.
Step 2: Improved microscopes
Leeuwenhoek's discovery interested other scientists but at that time microscopes were not good enough for his idea to be pursued further. However, in 1830 Joseph Lister, a British scientist, developed a microscope that magnified 1,000 times without distortion. With these improved microscopes scientists could observe in detail the behaviour of micro-organisms.
Step 3: Louis Pasteur's germ theory
In the 1850s Louis Pasteur, a French scientist,, was asked to help a brewing company find out why their vats of alcohol were going bad. Pasteur discovered that a particular microorganism was growing vigorously in the liquid, He developed a theory that these germs
(called germs because the microorganism is germinating or growing) were the cause of the problem. Pasteur killed these harmful germs by boiling the liquid. As a result, Pasteur became well known in France and he was called in by other industries who had similar problems. He found that micro-organisms seemed to he responsible not only for milk turning sour, but also for beer, wine and vinegar fermentation going bad.
Step 4: The battle between germ theory and spontaneous generation
Pasteur was an ambitious man who was keen to spread his theories further. In 1860 he got that opportunity when the French Academy of Science organised a competition for scientists to prove or disprove spontaneous generation.
The competition found Pasteur up against another leading French scientist —Pouchet.
Pasteur devised a series of ingenious experiments to prove that his theories were correct.
They succeeded triumphantly, and in 1861 Pasteur published his 'GERM THEORY'.

Pasteur's theories and experiments
THEORY
The air contains living micro-organisms
EXPERIMENT
He took sterile flasks out into the streets of Paris, opened them briefly, then sealed them again. Bacteria grew in them.
Microbes are not evenly distributed in the air
He repeated the experiment in various places around France including high mountains. The number of bacteria varied.
Microbes in the air cause decay
He filled two flasks; one with sterile air and the other with ordinary air. In the first there was no decay; in the second decay proceeded as normal.
Microbes can be killed by heating
He heated a material in a flask to make it sterile. He drove the air out, then sealed the flask. II remained sterile even 100 years later.
Step 5: Linking micro-organisms to disease
In his germ theory he also said: 'If wine and beer are changed by germs, then the same can and must happen sometimes in men and animals'. He speculated: could disease be caused by the same process as wine going sour or material decaying? Harmful germs get into a body, grow rapidly and cause the disease. He soon put this to the test. The French silk industry was being ruined because of a disease which was affecting their silkworms. Pasteur was called in to investigate. Here again he found that a particular micro-organism seemed to be causing the silkworms' disease.
Step 6: Proving the link between bacteria and human disease
Pasteur was a scientist - he carried out his early experiments with beer, wine and silkworms.
German doctor Robert Koch took up the challenge of applying Pasteur's ideas to human diseases. Koch conducted a series of experiments which proved once and for all that specific micro-organisms cause specific human diseases.
How important was Robert Koch? Robert Koch was born in Germany in 1843. As a doctor he became interested in Pasteur's germ theory. He bought a microscope and from 1875-78 he methodically studied anthrax, a disease which affects both animals and humans.

Koch's meticulous research so impressed people that the German government gave him a full-time job and a talented team of bacteriologists to continue his research. His methods were followed by other scientists in their search for the causes of disease.
Did germ theory help scientists cure diseases?
There was no doubting the importance of Koch's achievements. They inspired other scientists to search for the causes of many diseases.
Using Koch's methods, the causes of these diseases were identified very quickly. The starred ones were discovered by Koch and his team
1880 Typhus 1882 Tuberculosis* 1883 Cholera*
1886 Pneumonia 1887 Meningitis
1884 Tetanus
1894 Plague 1898 Dysentery
Koch's success spurred Louis Pasteur into action again. Pasteur was nationalistic and ambitious. France and Germany were deadly rivals at this time. France bad lost a bitter war to Germany in 1870-71 (the Franco-Prussian War). For the rest of his life, despite a severe stroke which paralysed the left side of his body, Pasteur applied himself to finding cures for disease. Pasteur knew he could not compete with Koch's work on his own. By this time he was powerful enough to be able to raise money from the French government and he gathered around him a team of doctors and vets to help with the research.
A new vaccination: chicken cholera
Pasteur had thought a lot about Jenner's work on smallpox. He was sure that smallpox was not the only disease that could be prevented by vaccination, but as he did not know how vaccination worked he had to continue his search by trial and error.
In 1879 Pasteur was researching chicken cholera because that was a disease which was troubling French farmers at the time. He extracted the germ which caused the disease and then started the process of trying to make a weak form of it. His team injected chickens with different strengths of culture to see if they worked.
During the summer of 1879, when the research was temporarily abandoned during the holidays, some chicken cholera solution was accidentally left unused in the laboratory. On returning to work, Charles Chamberland, a member of the team, happened by chance to inject chickens with these old germs that had been left exposed to the air throughout the

summer. This led to an amazing discovery. The old solution had IMMUNISED the chickens against the disease. Clearly, exposure to the air had weakened the germs. Pasteur called this method vaccination to show his debt to Jenner.
Vaccination number three: anthrax
Anthrax was a dangerous disease in animals and it could also kill humans. After patient experiment Pasteur's team announced that they had produced an attenuated (weak) culture which they believed could immunise animals against anthrax.
The Agricultural Society of Melun invited Pasteur to a public display to prove his ideas about anthrax were correct. This would have scared many scientists, but not Pasteur. In May 1881 he took up the challenge. 25 sheep were inoculated with weak virus and later with a stronger virus. By the end of the month they were all still healthy. These 25 sheep and another untouched 25 sheep were then given the strongest virus. The 25 inoculated sheep were fine, but the 25 untouched sheep died. This showed that the vaccine was a success.
Pasteur's critics found plenty to attack in his methods. Koch accused Pasteur of sloppy science, saying he did not accurately measure his anthrax vaccine. However, they could not argue with the results —the sheep were protected, He had risked public humiliation if the anthrax experiment had failed. Instead he succeeded triumphantly and his fame quickly spread.
Vaccination number four: rabies
Pasteur was now more confident of his method and in 1882 he turned his attention to rabies.
This disease is passed to humans from a bite by an infected dog. It was not a common disease in humans, but it caused certain death and the patient suffered terribly in the process. After two years of careful research Pasteur developed a rabies vaccine which worked. Soon people from all over Europe were flocking to Paris to be treated by Pasteur.