The discovery of germs led to the development of modern medicine. The germ theory displaced prior notions of disease such as the miasma theories that relied on airborne emanations from rotting organic matter.

For a microorganism to be considered the causative agent of a specific disease, it must meet four criteria listed by Koch. These are known as Koch’s postulates.

Origins

The germ theory of disease is a cornerstone of modern medical practice. It is widely regarded to be responsible for a great leap forward in health care, ranging from hygienic and surgical methods to vaccines and antimicrobial agents. Yet, known diseases remain formidable killers and new infectious strains are constantly emerging. Given this, a more cautious approach might be in order.

The emergence of germ theory swept away prior European and American notions of disease, including miasma theory—the airborne emanations of filth and decay resulting from the breakdown of organic matter—and contagionism, which held that one illness could transform into another or manifest differently in each individual based on his or her societal susceptibilities. It also helped give credence to sanitary reforms, such as water supply sanitation, food quality control, and sewage and waste treatment and disposal.

Pasteur developed his own techniques for preparing pure liquid cultures of bacteria that allowed him to positively identify disease-causing microorganisms. He then drew on these methods to develop protective vaccines.

But a germ theory of disease that focuses solely on the role of the causative microbe overlooks the many host factors that affect infection outcome. This includes the paradoxical finding that the same infectious agent can cause lethal fever in one person and asymptomatic infection in another, and that the optimal level of virulence from the pathogen’s perspective depends on whether or not it enhances between-host transmission.

Louis Pasteur

Louis Pasteur is often called the father of immunology, although Edward Jenner actually introduced vaccination to prevent infectious disease (Smith, 2011). The work of Pasteur and other scientists, such as Emil Roux, led to the development of vaccines against rabies and many other serious diseases. Pasteur believed that all infectious diseases were caused by living microbes and could be prevented by prophylactic vaccination and cured by therapeutic vaccination.

He also believed that the virulence of any microbe could be attenuated by passing them through a culture medium that had only trace amounts of vital nutrients. As a result, he kept his results and methods secret for 9 months before revealing them to the Academie des Sciences in October of 1885.

During this time, Pasteur fought long and hard battles to win acceptance of his germ theory. His explanation of fermentation and putrefaction challenged the ancient beliefs of spontaneous generation, as well as those of Girolamo Fracastoro and Marcus Antonius von Plenciz. He also had to fight to convince physicians that his discoveries about viruses, bacteria and other microscopic organisms could be applied to cure disease. Pasteur’s pasteurization process, which involved heating wine to kill unwanted microorganisms, was a key step in winning acceptance of his ideas. He used it to help fight cholera and other deadly infectious diseases, as well as to create his rabies vaccine.

Robert Koch

During the final decades of the nineteenth century, German physician Robert Koch led the advancement of germ theory, proving that microorganisms caused diseases such as anthrax, tuberculosis and cholera. His work revolutionized medical science and public health and established a framework for identifying infectious agents that still underlies aspects of contemporary medicine. He developed essential laboratory techniques and was the first to link specific bacteria with disease. He also formulated the four criteria that are now known as Koch’s postulates, which are still used today to determine whether a particular pathogen causes a disease.

The physical existence of germs had been proven by the microscope observations of Antoni van Leeuwenhoek in 1677, but the idea that they might cause disease was only slowly embraced by scientists in the 17th and 18th centuries. The germ theory superseded existing miasma (bad air from rotting organisms) and contagion theories of disease, and it remains a guiding principle for modern medicine.

Although the germ theory was widely accepted by 1900, it had many critics and a number of problems. Pasteur and Koch were rivals, and national and personal conflicts arose between French, British and German scientists. The germ theory fit well with the emerging philosophy of sanitary reform that was championed by Edwin Chadwick and other nineteenth-century public health workers; it was believed that disease rates could be reduced through improvements in water supply sanitation, food quality and sewage and waste treatment and disposal.

Girolamo Fracastoro

Girolamo Fracastoro is a Renaissance scholar who studied medicine, plants, philosophy, literature, music, mathematics, and astronomy. While he is known primarily for his medical writings, including Syphilis, sive morbus gallicus, a 1530 epic poem about the curse of syphilis on a shepherd who insulted Sol Pater, the sun god, he also wrote about contagious diseases and various natural subjects.

In 1546, he published De contagione et contagiosis morbis et curatione. It posited that diseases spread through direct contact or contaminated fomites, such as clothing or bedsheets; or at a distance, through seminaria, the seeds of contagion, which spread either by joining humors with their greatest affinity or by attraction, penetrating vessels.

In time, germ theory displaced earlier notions of disease, such as miasmas, the airborne emanations of filth and decay resulting from rotting organic matter. It also displaced the belief that one disease could transform into another or manifest differently in the body based on an individual’s susceptibility, which was thought to be determined by social factors such as moral values and social class. The bacteriologists of the 19th century would revive Fracastoro’s works, viewing him as an early forerunner of germ theory and the first to recognize that illnesses can be spread through microbial infection. He is sometimes referred to as “the father of modern epidemiology.” His works are available at the Internet Archive.