Germ theory of disease

The germ theory of disease is the idea that certain diseases are caused by the existence and activity of parasitic microorganisms, or germs, that have invaded a host from the outside. While an awareness of germs has existed since the 1600s, the relationship between germs and disease was not understood until two centuries later, when French chemist Louis Pasteur (1822–95) provided the first scientific proof that germs from bacteria actually cause diseases, rather than being spontaneously generated as a result of the disease.

Germ theory gradually gained acceptance, displacing prior European and American notions about disease, including the miasma theory—the belief that diseases were caused by miasmas, the airborne emanations of filth and decay resulting from rotting organic matter—and contagionism, which held that one disease could transform into another or manifest differently in the body based on an individual’s societal susceptibilities, such as moral values and social class. Knowledge of the germ theory radically changed medical perspectives and practices and continues to serve as the foundation of modern medicine.

Brief History

As part of Louis Pasteur’s job as professor of chemistry at France’s Université de Lille, he was charged with trying to find solutions to problems encountered by some of the local industries. He looked into the manufacture of spirits and found that bacteria were causing wine and beer to sour. He showed that by boiling and then cooling the liquid, a process now known as pasteurization, the bacteria could be destroyed and souring could be avoided.

Delving deeper, Pasteur began experimenting to identify the origin of the bacteria. He learned that they were introduced into the beverage-making process from external factors in the surrounding environment, such as air, rather than created as a consequence of the process. Pasteur’s findings were accepted by the Académie des sciences (French Academy of Sciences) in 1864.

The next year, as director of scientific studies at the École normale supérieure in Paris, Pasteur became involved in researching an epidemic that was harming the silkworm population, and thus the silk industry, in southern France. He discovered that a parasite was infecting certain silkworm eggs. By avoiding the diseased eggs, silk producers could revive their silkworm stocks and resume normal production. Pasteur’s findings not only saved the country’s silk industry but also convinced him that his theory of environment-based germs causing disease was correct. However, he still had to prove it.

To support his theory, Pasteur devised a series of tests in which flasks with curved necks were filled with a broth and left open to the air. He postulated that the bend in the neck would trap invading germs and prevent them from contaminating the liquid. The results supported his theory that germs are airborne and environmental as opposed to created by spontaneous generation.

German physician Robert Koch (1843–1910) expanded on Pasteur’s research by conducting experiments designed to determine whether specific bacteria caused specific diseases. To do so, he injected healthy mice with bacteria cultured from the tissue of diseased mice. In 1876, he was able to prove that the bacteria Bacillus anthracis was the cause of anthrax disease. With proof in hand, scientists then turned their focus toward identifying the germs behind other diseases, such as cholera, tuberculosis, smallpox, and rabies, and determining if they could be prevented through vaccines.

Overview

During the 1800s and early 1900s, scientists worked to solidify and advance the germ theory of disease as introduced by Pasteur and furthered by Koch. This period of time also saw the birth of epidemiology, which is the science of identifying the sources of an illness and tracking its spread through populations. The first example of epidemiology at work occurred in 1854, when English physician John Snow (1813–58) identified a specific water pump as the site of a germ contamination that had led to a cholera outbreak in the Soho area of London. He discovered the pump by tracing when and where individuals had first become ill. As epidemiologists and other scientists discovered that most illnesses could be attributed to microorganisms, another discipline emerged that continues to be a foundation for medical research and advances: microbiology, the scientific study of microorganisms.

While many disease-causing microorganisms were identified during that early period of discovery, scientists were still unable to create lifesaving therapies for infected patients. It was not until the introduction of antibiotics after World War II that there was a measurable decline in the incidence of some bacterial diseases, such as pneumonia and tuberculosis. The research advances were greatly driven by the development of the electron microscope in the 1940s, which enabled scientists to observe at a microscopic level the individual disease agents and their behaviors under various conditions. The introduction of methods to cultivate bacteria in a laboratory setting during the 1950s significantly expanded scientific knowledge about disease-causing germs and their activities, leading to the development of vaccines for such prolific diseases as polio, measles, mumps, and rubella.

The implications of microbiology research have expanded over the decades to influence fields as diverse as pharmaceutical development, food and beverage production, and agricultural pest control. In addition to serving as the basis for the creation of many disease vaccines, microorganisms are put to work as germ-breeding factories in order to produce drugs that otherwise could not be created in a laboratory, such as the human hormone insulin for the treatment of diabetes; the potent antiviral interferon, used to treat serious infections and some types of cancer; and enzyme products designed to help the body either form or dissolve blood clots. Knowledge of germ behavior is also used to hone the fermentation processes that create new food and beverage products at a mass manufacturing level, such as yogurt, pickles, and beer, derived from preexisting products such as dairy, cucumbers, and yeast. Similarly, increased understanding of germs led to the introduction of purification procedures for removing disease‐causing microorganisms from drinking water. In agriculture, modern scientists can reengineer bacteria to increase plant resistance to potentially harmful environmental factors such as destructive insects and frost.

While other scientists have contributed to the acceptance and advancement of the germ theory of disease over the centuries, it was the work of Louis Pasteur and Robert Koch that provided the scientific proof for the theory and opened the door for modern research dedicated to identifying, modifying, and manufacturing microorganisms in order to enhance people’s health and safety and help ensure the sustainability of some of the world’s resources.

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