LEEUWENHOEK
In Holland Anthony van Leeuwenhoek made microscopes as a hobby. He made over 500 "microscopes," of which fewer than ten have survived to the present day. In basic design, probably all of Leeuwenhoek's instruments - certainly all the ones that are known - were simply powerful magnifying glasses, not compound microscopes of the type used today. Antony van Leeuwenhoek learned to grind lenses, made simple microscopes, and began observing with them. He could magnify things 270 times his eyesight.
Leeuwenhoek succeeded in making some of the most important discoveries in the history of biology. His researches circulated, and opened a world of microscopic life to the awareness of scientists. He examined everything under his homemade lenses, even his own saliva and blood. In 1675 he looked at some rainwater under a microscope and discovered "animacules" that today we call microbes. Leeuwenhoek looked at animal and plant tissues, at mineral crystals and at fossils. It was he who discovered bacteria.

THE FATHERS OF BACTERIOLOGY

Long ago many people believed that evil spirits caused diseases.To the world of medicine, bacteriology was a miracle. The bacteria that were universally present in our world were finally being studied and understood. Two great men started all of this: Louis Pasteur and Robert Koch. These two scientists gave bacteriology a major role in medicine. Louis Pasteur and Robert Koch contributed with theories, ideas, and discoveries to the science of bacteriology.
Advances in technology meant that much more powerful microscopes could be made. These new microscopes helped scientists to look at bacteria. Louis Pasteur and Robert Koch were able to prove that some bacteria (germs) caused diseases. The study of bacteria led to the development of new medicines and vaccinations.

LOUIS PASTEUR
By experimenting with microbes, Louis Pasteur, a French chemist, developed the germ theory of disease. This theory says that a specific microbe causes every infectious disease. From this point of view, scientists have searched for the microbes that cause diseases in order to find out how to prevent and treat illnesses. Pasteur conducted a series of ingenious experiments that destroyed every argument supporting "spontaneous generation"
Pasteur carried out his research into germs in 1864.
His work on vaccines went on for about 4 years from 1877-1881.
He proved that the old idea that diseases start out of nothing (spontaneous generation) was inaccurate and that microorganisms cause disease.
He showed the importance of testing ideas in a scientific way. He discovered several vaccines that worked against certain illnesses in animals and one against rabies that also worked on humans.

ROBERT KOCH
Koch was a German scientist, influenced by Pasteur's work. In 1872 he began research into the microbes affecting animals and people. His careful research and observation using the microscope, photography and dyes led to a progress in the fight against two of the deadliest diseases of the late 19th century.
In 1882 he identified the bacteria causing tuberculosis. A year later, in 1883, he identified the bacteria causing cholera.
One of Koch's most important achievements was the formulation of his postulates. These are used to confirm if a microorganism causes a disease. First, the microorganism is observed in cases of the disease (The bacterium must be present in every case of the disease). Next, it is isolated and grown in a pure culture (The bacterium must be isolated from the diseased host and grown in pure culture). When the isolated microorganism is injected into a healthy animal, it must cause the disease (The specific disease must be reproduced when a pure culture of the bacterium is inoculated into a healthy host). Finally, the microorganism is observed and recovered from the new ill subjects (The bacterium must be recoverable from the experimentally infected host).