Germ theory is the understanding that tiny living things—too small to be seen with the unaided eye—can enter the body and cause disease . Such organisms are called germs. They include certain bacteria , viruses , fungi , and other microbes. Scientists in Europe developed germ theory roughly between 1870 and 1885. It forms a fundamental part of modern medicine .
Medicine before germ theory
Before the 1800’s, physicians maintained that illness was caused by an imbalance of four different humors (liquids) in the body. The humors were blood, black bile, yellow bile, and phlegm. This idea drew on texts attributed to the ancient Greek physician Hippocrates , who lived around 400 B.C. and is often called the father of medicine. Many people also thought that exposure to miasma, a foul-smelling vapor rising from decaying matter, could cause disease.
For centuries, however, observers had noticed that some diseases moved from person to person. One early public health measure was to separate sick people from healthy ones. For example, many communities kept people with the disease leprosy in separate colonies. Another early health measure was to hold a ship outside of port for 40 days if officials suspected it carried infection among its passengers. This practice, which became common during the Black Death in Europe in the 1300’s, gave rise to the word quarantine , which comes from the Latin quadraginta, meaning forty.
In 1634, the Dutch scientist Anton van Leeuwenhoek viewed tiny creatures under his microscope. He called these microbes animacules. He demonstrated that they could be found in the air, water, and soil. However, few people believed that such microbes were responsible for disease.
The development of germ theory
Many scientists contributed to the development of germ theory. Significant contributions came from the French microbiologist Louis Pasteur and the German bacteriologist Robert Koch .
Pasteur showed that some diseases are caused by microscopic germs that multiply in the body. He also demonstrated that a vaccine made from such germs could protect animals from certain diseases. Pasteur demonstrated the efficacy of his vaccine against the disease anthrax in farm animals in 1881. In 1885, he demonstrated his rabies vaccine on a boy bitten by a rabid dog. With this work, Pasteur convinced many people that germ theory was valid.
In the 1880’s, Robert Koch developed a set of logical steps to identify disease-causing germs. These steps, now known as Koch’s postulates, are still used by bacteriologists . He found that a particular germ could be shown to cause a particular disease under the following conditions. First, a researcher must be able to consistently obtain the germ from the tissue of animals with the disease. Second, the researcher must be able to culture (grow) the germ in a laboratory. Third, the researcher must be able to cause the same symptoms by introducing the germ into a healthy animal. Finally, the researcher should be able to obtain the same germ from the newly sick animal. Koch identified the germs that cause anthrax , cholera, and tuberculosis.
Acceptance of germ theory
By the mid-1880’s, many scientists were convinced that germ theory explained the cause of most diseases. Acceptance of germ theory was not universal, however. Certain diseases, such as typhoid , did not appear to fit the theory. Researchers later learned that typhoid can be spread by carriers, who do not suffer from the disease but continue to spread germs to others. The fraudulent or mistaken identification of germs that caused diseases such as cancer or scurvy also challenged support for germ theory.
In the decades following Koch’s and Pasteur’s discoveries, scientists continued to identify microscopic organisms that caused many deadly diseases. Bacteria and viruses proved the most common disease-causing germs, but others include fungi, protozoans , and worms . In medicine today, such disease-causing agents are called pathogens.
Today, germ theory forms the foundation of the scientific understanding of infectious disease. Such diseases occur when an organism or other agent gains entry to the body and reproduces itself. However, not all diseases are infectious. Noninfectious diseases constitute a broad group of illnesses not caused by germs. These include many cancers, inherited diseases, metabolic diseases, and environmental diseases.
