Today, it’s difficult to comprehend the “germ theory” revolution in medicine and the significance that animal research played in its development. It’s hard to believe that people used to believe that unpleasant odors might cause disease or that “evil spirits” could make someone sick. We’ve also forgotten how uncommon it used to be for parents to see all of their children grow up to be adults. Even so, it’s only been around a century and a half since Robert Koch made the discoveries that led Louis Pasteur to describe how microscopic organisms known as bacteria might infect the body and cause sickness.
Koch proved decisively in the last decades of the nineteenth century that a certain germ could produce a specific disease. By experimenting with anthrax, he was able to do this. Koch analyzed the blood of anthrax-stricken cows under a microscope. He saw rod-shaped germs and assumed these were the source of anthrax. Koch inoculated mice with anthrax-infected cow blood, and the mice contracted anthrax as well.
As a result, Koch devised a set of four criteria for determining if a given germ is responsible for a specific disease. Koch’s Postulates are a set of criteria that are still used today. Postulate 3, “The disease must be generated when a pure culture is implanted into a healthy, susceptible host,” is essential to these conditions.
Even today, with all of contemporary science’s breakthroughs, proving that a specific germ is responsible for an illness without the use of laboratory animals is impossible.
Definition of Germ Theory
The germ theory of disease is based on the idea that many diseases are caused by microbial infections that can only be seen under extreme magnification. Bacteria, viruses, fungi, and protists are examples of such microorganisms. Although disease is caused by the growth and productive replication of microorganisms, environmental and genetic factors can predispose a host to infection or alter the severity of the infection. In a host that is immunocompromised (e.g., due to AIDS or advanced age), an infection may have more severe consequences than in people who are fully immune.
Who Was the First to Propose the Germ Theory of Disease?
The idea that diseases may be transferred by “seed-like organisms” was first proposed by Girolamo Fracastoro in the 1500s, and diseases were classified according to how they could be transmitted. Later, in the early 1800s, Agostino Bassi conducted a series of studies that proved that an illness that afflicted silkworms at the time was caused by a parasite. Microorganisms, according to Bassi, are also responsible for sickness in people and animals. Bassi’s work influenced Louis Pasteur, who is credited with developing the germ theory of disease after proving the link between microbes and disease in his studies.
Pasteur’s Experiments (Louis Pasteur’s Experiments)
Pasteur was the first to show that disease was produced by microorganisms in the environment, not by the air itself, as the popular theory at the time suggested (Miasma theory). This was accomplished by exposing freshly boiled broth to air under the conditions shown in the image below.
To avoid exposure to particles, a filter was placed in the tank containing the growth media. Without a filter, the vessel containing the growth medium was exposed to ambient air. A long tube was used to expose the vessel containing the growing media to room air, preventing dust particles from entering (as shown below).
Pasteur discovered that organisms only grew in the broth when it was exposed to room air without a filter. As a result, he concluded that the organisms did not come from the broth or the air itself, but rather from airborne particulates. Pasteur’s germ theory of disease was eventually expanded further by later scientists such as Robert Koch.
The Evolution of Germ Theory
Early Disease Theories
In ancient Greece, disease was supposed to spread through infectious “seeds” in the air or food products, rather than through direct contact with other infected people. Furthermore, such seeds could become lodged in a person’s body, resulting in a disease relapse at a later date. Scholars of the Middle Ages (e.g., Girolamo Fracastoro) expanded on this concept, adding that disease might be spread through direct or indirect touch as well as distant distances. The idea that disease-causing seeds may be latent was also maintained, and many diseases were classified according to how long they had been dormant.
The idea that disease was caused by spontaneous generation was experimentally debunked during the 1600s, notably through the work of Francesco Redi. The following were Redi’s experimental findings:
Jar 1: Without a cover, meatloaf and an egg are exposed to the air.
The egg and meatloaf were covered in maggots as a result.
Jar 2: Meatloaf and an egg in a jar carefully sealed with a lid.
There were no maggots found.
Jar 3: Meatloaf and an egg in a jar with no lid but gauze over it.
Maggots on top of the gauze as a result.
Based on these data, Redi came to the conclusion that the maggots were only located on accessible surfaces, proving that spontaneous generation was not the case.
Another early microbiologist from the 1600s was Anton van Leeuwenhoek, who invented the first microscope and was the first to directly see the presence of microorganisms (which he referred to as “animalcules”). Richard Bradley proposed the idea that sickness was caused by animals that could only be seen via a microscope in the 1700s.
Marcus Antonius von Plenciz later backed up this hypothesis, writing a book outlining how diseases produced by tiny creatures could be divided into those that were contagious but did not generate epidemics, and those that had both characteristics. Von Plenciz went on to say that minute organisms are everywhere.
Theory of Miasma
Until the germ theory of disease was adopted in the 19th century, the dominant idea was known as “miasma theory,” which meant “pollution” or “poor air.” According to the miasma idea, disease was caused by the breakdown of organic materials, which resulted in a noxious mist containing disease-causing chemicals. Furthermore, inhaling foul-smelling air coupled with tainted drinking water, unhygienic environments, and air pollution could lead to disease.
Figures Who Shaped the Evolution of Germ Theory
Ignaz Semmelweis, John Snow, and Robert Koch, in addition to the individuals mentioned above, were essential figures in the creation of the germ theory of illness and the transmission of germs within a population.
Ignaz Semmelweis
In the mid-nineteenth century, obstetrician Ignaz Semmelweis noticed a higher mortality rate due to puerperal fever in women who gave birth later in the day with the help of doctors and medical students compared to women who gave birth earlier in the day with the help of midwives. During his inquiry, he discovered that the doctors and medical students who assisted women in giving delivery had previously performed autopsies.
According to Semmelweis, puerperal fever was caused by a sickness that was communicated to pregnant women via cadavers in autopsy rooms. Following this insight, Semmelweis established required handwashing in a chlorinated solution of lime water before to aiding with births, and reduced the birthing death rate from 18 percent to 2.2 percent. Despite the success of obligatory handwashing, society at the time rejected Semmelweis’ theory.
John Snow
John Snow was one of the first to publish an epidemiological study describing the transmission of cholera via the fecal-oral route, despite the lack of a fully developed germ theory of disease. In addition, he disputed the miasma idea in his description of cholera transmission, stating that cholera was most likely transmitted by inadvertently ingesting feces from a sick individual. He went on to say that the cholera organism may cling to the lining of the intestine, reproduce, and infect the next host. Based on his epidemiological research of the London cholera outbreak of 1854, Snow recommended that water be filtered and boiled before drinking.
During his examination of the cholera epidemic, he discovered that a public water pump was the cause of the disease, and that once disconnected, it helped to put an end to the pandemic. Using a dot map, Snow demonstrated the link between cholera cases and the water pump. During his research, he discovered that the pump was supplying sewage-tainted water, which people were subsequently consuming. Furthermore, the pump was discovered to be next to an inactive cesspit, which was spilling feces into the pump’s water supply. Despite the soundness of Snow’s findings, the concept of fecal-oral illness transmission was met with popular opposition.
Nonetheless, his work on characterizing cholera transmission is still regarded as one of the most important events in public health history.
Robert Koch
Robert Koch created the basic scientific conditions for demonstrating that each disease is caused by a distinct microbe, building on Louis Pasteur’s early work and the germ theory of disease. Koch’s Postulates are a set of requirements based on Koch’s work with anthrax isolated from infected hosts. The four rules that make up Koch’s Postulates are as follows:
- The bacterium must be detected in all people who have been diagnosed with the disease, but not in healthy people.
- The diseased person’s microbe can be isolated and cultured in culture.
- The cultured microorganism should induce sickness when injected into a healthy person.
- After that, the microbe must be extracted from the experimental host and confirmed to be identical to the original microorganism.
Limitations of Koch’s Postulates
While Koch’s Postulates were created as general guidelines for identifying infectious causes of disease, they had several inherent limitations that were not addressed at the time. One of these restrictions is that, while some of the postulates could be verified, viruses could not yet be cultivated in the 1800s. While it appeared that an infectious agent was to blame for certain diseases, the lack of technology to isolate and culture viruses meant that not all of Koch’s Postulates could be fulfilled.
Furthermore, the third postulate states that the experimental host “should,” not “must,” develop disease. This is due to the possibility of asymptomatic carriers, immunity, and genetic resistance.