Bacteria

After the momentous medical breakthroughs of Vesalius and Harvey came the 17th-century discovery of one of the human body's greatest enemies: bacteria. This discovery eventually led to the realization that exposure to certain microorganisms could cause disease. It also prompted new theories of antiseptics that sharply lowered mortality rates from surgery.
Antoni van Leeuwenhoek, a part-time janitor and haberdasher working in Delft, Holland, discovered bacteria and other microorganisms using a microscope that he built himself. Through the influence of a friend, a Dutch physician, he was invited to write letters to the Royal Society of London—a group dedicated to the advancement of science. These letters were translated from Dutch into English and published in the society's journal Philosophical Transactions.
Leeuwenhoek's most famous letter was published on March 16, 1677. In this letter he described looking at a drop of rainwater through his microscope. The drop was taken from a tub where it had been allowed to stand for several days. To his amazement, he saw exceedingly tiny animals, known today as protozoa, swimming in the water. He also observed other equally small animals that did not move at all, now known as bacteria. No one at the Royal Society knew anything about these little creatures, which Leeuwenhoek called animalcules. At the request of the stunned members of the Royal Society, several of the most respected citizens of Delft were asked to verify Leeuwenhoek's microscopic findings. They did so, and in 1680 Leeuwenhoek was elected a fellow of the prestigious Royal Society.
Later discoveries extended the significance of Leeuwenhoek's work, especially the superb finding by the German scientist Robert Koch in 1876. Koch found that the microscopic anthrax bacillus could actually cause a fatal human disease. Until Koch's discovery, many scientists thought it absurd that microscopic creatures could harm much larger animals, such as humans. In 1882 Koch showed that another kind of bacterium, the tubercle bacillus, caused tuberculosis, a discovery for which he won the Nobel Prize in 1905.
Unlike Koch, who was a country physician when he made his epochal discoveries, the French chemist and biologist Louis Pasteur disliked physicians so much that he would not have them as workers in his laboratory. Despite his disdain for physicians, he was deeply fascinated by diseases of various kinds. Pasteur discovered that putrefaction (a decomposition of organic substances) is caused by microorganisms that float in the air. Pasteur learned that he could prevent putrefaction by subjecting organic substances to moderate but not extreme heat, a process known as pasteurization.
In 1865 Joseph Lister, an English surgeon, read of Pasteur's research on putrefaction. Lister recalled that whereas simple bone fractures invariably healed, compound fractures (bone fractures bursting the skin) almost always began to putrefy. Lister was certain that this dangerous, infectious process was caused by the wretched microorganisms that Pasteur had described. To test his theory, Lister covered his patients' compound fractures, previously exposed to the air, with linen strips soaked in carbolic acid. He believed the carbolic acid might exterminate the airborne microorganisms.
Lister treated compound fractures and open surgical wounds with carbolic acid for nine months. During this time, he did not observe a single infection in his surgical ward. The results of his experiments, published in 1867, gave rise to antiseptic surgery. Although Lister's antiseptic technique initially encountered resistance from other physicians, it soon became widely accepted, and deaths due to infections in the operating room plummeted.
source: encarta enyclopedia