Germ theory, in medicine, the theory that certain diseases are caused by the invasion of the body by microorganisms, organisms too small to be seen except through a microscope. The fact that many kinds of diseases are related to microorganisms was unknown until relatively recently. Before the time of Pasteur, effective treatments for many diseases were discovered by trial and error, but the causes of the diseases were unknown.
The realization that yeasts play a crucial role in fermentation was the first link between the activity of a microorganism and physical and chemical changes in organic materials. This discovery alerted scientists to the possibility that microorganisms might have similar relationships with plants and animals specifically. That microorganisms might cause disease. This idea was known as the germ theory of disease.
The germ theory was a difficult concept for many people to accept at that time because for centuries disease was believed to be punishment for an individual’s crimes or misdeeds. When the inhabitants of an entire village became ill, people often blamed the disease on demons appearing as foul odors from sewage or on poisonous vapors from swamps.
Most people born in Pasteur’s time found it inconceivable that ” invisible” microbes could travel through the air to infect plants and animals or remain on clothing and bedding to be transmitted from one person to another. But gradually scientists accumulated the information needed to support the new germ theory.
In 1865, Pasteur was called upon to help fight silkworm disease, which was ruining the silk industry throughout Europe. Years earlier, in 1835, Agostino Bassi, an amateur microscopist, had proved that another silkworm disease was caused by a fungus. Using data provided by Bassi, Pasteur found that the more recent infection was caused by a protozoan, and he developed a method for recognizing afflicted silkworm moths.
In the I 86Os, Joseph Lister, an English surgeon, applied the germ theory to medical procedures. Lister was aware that in the 1840s, the Hungarian physician Ignaz Semmelweis had demonstrated that physicians, who at the time did not disinfect their hands, routinely transmitted infections (puerperal, or child – birth, fever) from one obstetrical patient to another.
Lister had also heard of Pasteur’s work connecting microbes to animal diseases. Disinfectants were not used at the time, but Lister knew that phenol (carbolic acid) kills bacteria, so he began treating surgical wounds with a phenol solution. The practice so reduced the incidence of infections and deaths that other surgeons quickly adopted it. Lister’s technique was one of the earliest medical attempts to control infections caused by microorganisms. In fact, his findings proved that microorganisms cause surgical wound infections.
The first proof that bacteria actually cause disease came from Robert Koch in 1876. Koch, a German physician, was Pasteur’s young rival in the race to discover the cause of anthrax, a disease that was destroying cattle and sheep in Europe.
Koch discovered rod -shaped bacteria now known as Bacillus anthracis in the blood of cattle that had died of anthrax. He cultured the bacteria on nutrients and then injected samples of the culture into healthy anima ls. When these animals became sick and died, Koch isolated the bacteria in their blood and compared them with the bacteria originally isolated. He found that the two sets of blood cultures contained the same bacteria.
Koch thus established a sequence of experimental steps for directly relating a specific microbe to a specific disease. These steps are known today as Koch’s postulates. During the past 100 years, these same criteria have been invaluable in investigations proving that specific microorganisms cause many diseases.