The History of Germ Theory

Before the 1800s, it was not widely understood that microscopic organisms cause disease. Learn how germ theory forever changed the way we understand illnesses and the body.

What is Germ Theory?

Germ theory is essentially the understanding that microscopic organisms cause disease. While it was first proposed in the 16th century, germ theory was developed and proved in the mid-1800s. Before that, from the time of Hippocrates in the 5th century BCE, Western medicine was centered around the *humoral theory* and each individual having a particular *humoral institution*.

*Humoral* is derived from ‘humor,’ in this context meaning ‘fluid,’ and the human body was believed to consist of four humors: black bile, yellow bile, blood, and phlegm. Proper humoral balance translated to perfect health, whereas an imbalance of the humors resulted in disease.


In contrast to humoral theory, germ theory established that diseases were separate from the body, and it revolutionized the medical field and the understanding of disease. Its implications were so significant that it brought about an elevation in the social status of physicians and of medical research and practice.

Spontaneous Generation

One of the theories on disease origination that had dominated the medical field since the times of Ancient Greece was that of spontaneous generation, first suggested by Aristotle in 350 BCE. This theory proposed that living organisms could arise from non-living matter like soil and water, and that it was impossible to confirm whether microorganisms found in non-living matter were the cause or the product of decomposition.

Some fascinating examples of *spontaneous generation* include that garbage in the streets created rats, scorpions could be manufactured using bricks and basil leaves, and crocodiles in Egypt were created by mud with the sun acting as catalyst.

It wasn’t until the 1870s when Louis Pasteur proved that decomposition was indeed caused by microorganisms, that the concept of disease also being the result of microorganisms became a possibility.

Early Germ Theories

The first to describe the concept that diseases could be spread by ‘seed-like entities’ was Girolamo Fracastoro, an Italian physician, in the 1500s, who proposed that disease could be spread in 3 ways: by direct contact; by carriers such as dirty clothes; and through the air. Fracastoro was the first to scientifically explore and understand the nature of germs, infection, contagion, and disease transmission.

In the early 1800s, Agostino Bassi, an Italian bacteriologist, conducted a series of experiments investigating the silkworm disease which was plaguing Italy and France. Following 25 years of research, he was able to demonstrate that the disease was contagious and was caused by a microscopic, parasitic fungus, concluding that it was transmitted among the silkworms through contact and by infected food. Bassi proceeded to theorize that disease in humans and animals was also caused by microorganisms, though never proved it.

Louis Pasteur

Louis Pasteur was a French chemist and microbiologist, and the first to experimentally prove the connection between germs and disease in the 1860s. Using swan-necked flasks that he had created, he boiled broths and realized that the liquid remained free of microbes until he broke the neck of the flasks and air particles were allowed to drift in.

Once the air was allowed in, Pasteur noticed under a microscope that microorganisms were growing in the liquid. In other words, keeping sterilized water in a sealed flask prevented germs from entering it, whereas exposing it to air would encourage microorganisms to breed again.

Pasteur’s conclusive experiments served to debunk the theory of spontaneous generation – essentially that life arises from non-living matter – that had been dominating the medical field from ancient times. His pioneering studies revolutionized the understanding of diseases and their etiology and laid the foundation for modern day treatment.


Robert Koch

While Louis Pasteur proved that germs were indeed linked to disease in the 1860s, German scientist Robert Koch – one of the founders of bacteriology – developed sophisticated laboratory techniques in the 1870s to prove that specific germs cause specific diseases. Koch observed bacterial growth on microscope slides, and his first important discovery was anthrax which was killing much livestock at the time.

Koch determined the 4 criteria to determine whether a specific organism causes disease, known as ‘Koch’s postulates:’firstly, a specific microorganism is always associated with a given disease. Secondly, the microorganism can be isolated from the diseased animal and grown in pure culture in the laboratory. Thirdly, the cultured microbe will cause disease when transferred to a healthy animal. And finally, the same type of microorganism can be isolated from the newly infected animal.

After extensive experiments, Koch also discovered the bacteria that cause septicemia, tuberculosis, and cholera, and his methods enabled others to identify many more pathogens in the years following. In 1905, Koch received the Nobel Prize for Physiology or Medicine for his work on tuberculosis.


Joseph Lister

In the early 1860s the mortality rate of surgical patients due to post-operative infections was estimated to be close to 50%. This staggering statistic might make more sense if one considers that, in those days, doctors arrived in the operating theatre in their street clothes without even washing their hands.

In 1864, Joseph Lister, a British surgeon, came across Louis Pasteur’s work on germ theory and sensed that germs could explain wound sepsis. As a result, Lister began spraying carbolic acid on open fracture wounds, which were almost always treated with amputation.

In 1865, Lister was successful at healing wounds without infections. From then on, he created a strict protocol for sterilizing everything before and after surgery, which serves as an inspiration to surgeons today. His discovery revolutionized surgery and drastically reduced post-operative deaths due to infections. Lister was a pioneer in preventative medicine and his name is associated with antisepsis – there is even a mouthwash named after him.


Public Awareness and Prevention vs Treatment

The development of germ theory and the discoveries of Pasteur, Koch, and Lister required a new public awareness of how germs cause diseases and of how germs spread from one person to another. In the late 1800s, new guidelines were issued regarding personal and home hygiene, cooking, heating, and plumbing, some which we still use today, and which were greatly emphasized with the outbreak of COVID-19.

Germ theory revolutionized the understanding of disease and provided the necessary knowledge to limit contagion and prevent disease outbreaks. It is essential to note, however, that while germ theory provided answers regarding the cause of disease, it did not necessarily provide answers toward the treatment of those diseases. In other words, the identification of a disease cause does not automatically lead to a cure for that disease.

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