Mid-way through the nineteenth century, people still believed in the disease theory of miasma. Miasma was a type of “bad air” that was thought to arise from the ground and cause epidemics. This theory was mainly promoted by plague doctors and it served as a way to explain diseases such as the bubonic plague, cholera, and chlamydia. Sadly, this theory was accepted for hundreds of years by both commoners and royalty. It wasn’t until Louis Pasteur came along and proposed the germ theory of disease that the fields of modern microbiology and medicine were born. Pasteur proved that “bad air” wasn’t the source of disease, but rather that microorganisms were the cause of infectious diseases. Although Pasteur’s theory was correct, many members of the community in Europe discarded his theory as the belief of a paranoid scientist. It was not until the world of academia studied Pasteur’s work that this theory began to be taken seriously.

Louis Pasteur was a critical scientist in many fields of science, including biology, microbiology, and chemistry. In 1856, light microscopes had been developed and Pasteur became interested in the process of fermentation.¹ Although it proved to become a revolutionary finding, Pasteur first encountered this process as a problem. A manufacturer of beetroot alcohol presented a problem where his batches were of poor alcoholic quality because the alcohol had an acidic taste, and the fermentation gave off fetid odors.² These effects were critical to the manufacturing of alcohol and in order to figure out how alcohol was made, he needed to approach the problem a different way. Since Pasteur thought as both a chemist and a biologist, he decided to bring a microscope to investigate and see if he was able to observe fermentation taking place. This new approach was an incongruous thing to do for chemists, but thanks to Pasteur’s thinking, he was able to see that small round organisms, known as yeast, contributed to alcoholic fermentation.³

After examining the process of fermentation more closely, he was able to determine that the acidic taste came from the build-up of lactic acid, and the fetid odors developed thanks to hydrogen molecules attaching to the nitrites of the beets.⁴ As Pasteur noticed these traits, he began to see a pattern. He first realized that in defective batches, the shape of the organism was in the form of an elongated rod rather than a small sphere.⁵ Pasteur realized that this rod-shaped organism, bacteria, was present in most of the surfaces that he had analyzed with his microscope, and he then turned to test the theory that would become known as spontaneous generation. This theory hypothesized that living organisms arose from inorganic matter. An example for this theory would have been that if you put a sweaty shirt and wheat in a container, in twenty-one days mice would be created.

In order to disprove this theory, Pasteur first had to stick with what he knew best, and that was microorganisms. These organisms also fell into the category of spontaneous generation, and so Pasteur sought to disprove this theory. The experiment he conducted had to do with two different containers. Both had a sterile nutrient-rich broth, but one had a wide and short neck, while the other container had an elongated and skinny neck which was S-shaped.⁶ After leaving both of the containers exposed for a day or two, he noticed that the container with the wider neck had the most bacterial growth, but when it came to the container with the skinny and elongated neck, there was no bacterial growth. This occurred because rather than being spontaneously generated, the bacteria were in the air. It was the only logical explanation that would explain why all the bacteria were trapped in the neck of the container rather than in the broth. But although Pasteur’s findings were backed up by proof, many remained skeptical. It wasn’t until Robert Koch came along that others began to take Pasteur’s theory debunking spontaneous generation more seriously.

Robert Koch was a brilliant microbiologist who decided to expand on the findings of Louis Pasteur. So, he devised his own experiment. After he studied Bacillus anthracis, a bacterium that caused the disease known as anthrax, he devised an experiment that dealt with the infectivity of Bacillus anthracis.⁷ He began by isolating the bacterium in a culture and proceed to experiment with it in mice. He injected the bacterium into a mouse and waited to see what happened. Fifty hours after the injection, death had taken place, and the organism didn’t show any vitals.⁸ Shortly after death, Koch extracted blood from the dead mouse and cultivated another culture, where the bacteria grew. After the growth, Koch repeated the experiment and extracted the bacterium from the newly grown culture. He proceeded to inject another mouse and afterward he saw the same symptoms and a similar time of death to the first mice.⁹ As he was conducting this experiment, he paid close attention to the tissues of the dead mice and came across something peculiar. He saw that every single mouse that died from anthrax had the same rod-shaped bacteria, Bacillus anthracis, although he wasn’t certain on how the disease was transmitted or how the bacteria managed to survive between hosts.¹⁰

Koch then continued his thirst for knowledge on infectious diseases by experimenting with tuberculosis in 1884. He was able to determine four postulates:

1. The putative organism must be constantly present in diseased tissue.
2. The organism must be isolated in pure culture.
3. The pure culture must induce disease when injected into experimental animals.
4. The same organism must be isolated from these diseased animals.¹¹

After Pasteur’s experience with fermentation and spontaneous generation, Pasteur felt ready to expand his knowledge of what he knew of microbiology. In the spring of 1865, Pasteur decided to undertake a project that dealt with a silkworm disease. This disease had caused devastating harm to the silkworm industry in southern France and delayed the production of silk.¹² Today, this disease is known as pébrine or pepper disease. Some physical characteristics of this disease were the tiny black spots on the skin of the worms and a change of color of the silkworm eggs.¹³ Rather than being yellowish, the shell of the infected silkworm eggs had a grayish tone. As Pasteur became intrigued with this problem, he looked deeper into the disease that not only affected the silkworm at a physical level but must have also affected them at a physiological level as well. When the infected silkworm hatched, it would stay put and look for a quiet place, unlike its healthy counterparts who would look for leaves to feed on immediately after hatching.¹⁴ This disease caused the silkworms to die at an alarming rate and they also weren’t able to spin a cocoon of silk that could be used in the textile industry, thus, creating an alarming problem when it came to the production of silk.¹⁵

Pasteur soon began to take notes of these accounts and saw a pattern. He saw that any of the silkworms who carried the tiny black spots were infected and those who were free from these spots were relatively healthy worms. Soon he ordered for any silkworm with spots to be stopped from breeding, in order to stop this disease once and for all. But as spring came, he realized he had made a dreadful mistake in his experiment. After the silkworms hatched, he saw that the eggs that came from what he thought were healthy silkworms, had infected silkworms.¹⁶ This meant that Pasteur had to rethink his idea of the type of disease that was affecting the silkworms. After examining the silkworm generation that hatched in the spring, he noticed that some of the silkworms that didn’t have the black spots died while others who had the black spots survived.¹⁷ This was a mystery that puzzled Pasteur for three years.

Pasteur concluded the possibility that they were suffering from two different diseases rather than just one.¹⁸ These two diseases were caused by separate microorganisms.¹⁹ One of the microorganism lived in the silkworm while the other lived in the black globules or spots that were present on the skin of the silkworm. Soon, Pasteur realized that both of these diseases were spreading to other living silkworms from a single infected silkworm. After making this realization, all Pasteur had to do was to figure out how this disease was able to spread from an infected organism to a healthy organism. He found that the uninfected silkworms had one thing in common: mulberry leaves. This leaf was conventionally used to feed silkworms since it is the only kind of leaf they will eat.²⁰ Pasteur then conducted a test to see if his theory was true, and indeed it was. It was revealed to him that if an infected silkworm laid dropping on the leaf and an uninfected silkworm ate it, the uninfected silkworm would contract the disease.²¹ One disease was recognized to be flacherie, an infectious disease that attacked the intestines of the silkworm and caused diarrhea. This problem was soon resolved by distinguishing whether a silkworm was suffering from pébrine or flacherie, and then stop them from breeding. And this was how Pasteur was able to save the silk industry from certain doom.

After deducing that these two diseases originated from two separate microorganisms, Pasteur became interested to see if all diseases were caused by environment-based germs. After analyzing data for a period of ten years, Pasteur had sufficient evidence to back up his claim that microorganisms caused diseases. He named his theory the germ theory of disease and it was a revolutionary step in the world of both microbiology and medicine. After his theory was published, people began to believe that microorganisms caused disease rather than miasma and soon enough the miasma theory was soon forgotten; but Pasteur still wanted to surpass Koch in the field of immunology and microbiology. So, he made it a challenge to himself to find a cure for rabies.

But before he was able to publish his germ theory of disease, there was an indirect contributor to the germ theory. This man was Edward Jenner, a country surgeon in the late eighteenth century who was born in Berkeley, England, nearly sixty years before Pasteur developed the germ theory of disease.²² His major contribution to germ theory was that he was the first person in the records to make a vaccine against smallpox. Smallpox was caused by a virus that entered the human respiratory tract and incubated for a period of twelve days. After roughly twelve days, the patient encountered symptoms such as fever, malaise, muscle aches, headaches, and pustules that covered the face and the extremities. After the disease had settled, the pustules turned into scabs that permanently disfigured their victims for the rest of their lives, if they even managed to survive this deadly disease.²³

Jenner had had a close life-or-death encounter when he was in his youth. At the age of eight, his school had fallen victim to an epidemic of smallpox and Jenner soon contract smallpox. After he was diagnosed, Jenner went through a process known as variolation, where he was bled until his blood was “thin.” This process was repeated until he had the appearance of a skeleton.²⁴ At this point, Jenner was inoculated with a live virus of smallpox, and surprisingly, Jenner survived and was able to continue his studies.²⁵

Jenner grew up, and he became a country surgeon. And he encountered a peculiar scenario. He overheard a milkmaid say she would never have to suffer from smallpox because she had already contracted cowpox, a weakened version of smallpox that affected cows.²⁶ This gave Jenner an idea of injecting a perfectly healthy human with cowpox to see if it indeed protected a person from smallpox. But he had to wait for the perfect time to conduct this experiment. The opportunity soon arose in the spring of 1776, when an outbreak of cowpox had taken place near Berkeley, England. From here, Jenner extracted blood from a milkmaid who was suffering from cowpox, and by May 14, he had inoculated a boy who had the exact same age as Jenner when he had suffered from the smallpox virus. After being injected with cowpox, the eight-year-old boy, James Phipps, suffered a mild fever for two days but survived. Afterward, the boy was exposed to the smallpox virus not once but twice, and he still didn’t get sick.²⁷ This proved to be a revolutionizing technique that would affect millions of human lives in the following centuries. After this, Jenner was recognized as a pioneer and the father of immunology.²⁸ But even though he made this significant discovery, it was rejected by his peers, and Jenner was even taunted by members of the Protestant Church. The would have drawings of cows growing on people’s skin after they had been vaccinated with the cowpox virus. Although his work wasn’t taken seriously, Pasteur recognized Jenner’s potential and started an experiment of his own: to conquer rabies once and for all.

Pasteur began his quest to eradicate rabies by creating a vaccine for it. In the nineteenth century, rabies was fairly common in developed countries. Rabies was essentially a virus that targeted the nervous system of an organism and it was normally transmitted through a bite from a rabid animal.²⁹ But there was only one catch to this story; Pasteur wasn’t able to see this microorganism through his microscope. Although all stakes were stacked against him, Pasteur was able to cultivate this invisible agent of disease in his laboratory, and he conducted an experiment to see if he could copy what Edward Jenner had done.

He first found a dog who was suffering from rabies and extracted a sample from the dog’s spinal cord. In order to do so, Pasteur had to perform trepanation on the dog. After extracting the sample, they suspended the samples in dry, sterile air with caustic potash to prevent putrefaction and allowed oxygen to weaken the virus over a period of two weeks.³⁰ After the two weeks, new dogs were inoculated with emulsions from the spinal cords that had been dried for fourteen days. As this process continued, it proved to be effective in dogs and caused an immunity to rabies, but this didn’t mean that it was meant for human use too. Pasteur created a vaccine for rabies in dogs, but his hands would have been tied for the scenario that presented in front of him in July of 1885. A boy by the name of Joseph Meister had been bitten by a rabid dog and his parents were pleading for Pasteur to use the vaccine on the innocent child.³¹ This placed Pasteur in a risky situation because the vaccine for rabies was used to prevent rabies, not treat rabies. If this didn’t work, Pasteur was putting his reputation and livelihood at risk by evening thinking of performing this experiment on a nine-year-old boy.

Ultimately, Pasteur opted to try out the experiment on July 7, 1885, by treating young Meister with the rabies vaccine.³² As the days passed, little Joseph was beginning to feel better and soon enough, he was on his way to his hometown. Luckily for Pasteur, his bet had given fruit to a new vaccine that could treat rabies, even after the virus was in the organism. This proved to be an extraordinary finding because, over the course of fifteen months after Meister was treated for rabies, two-thousand four-hundred ninety lives had been saved by the vaccine.³³ Today, this vaccine is still being used in circulation and it has saved millions of lives from a certain death sentence.

Although only three examples of Pasteur’s work in science was mentioned, he was able to do other numerous applications that helped industries. For example, after determining that spontaneous generation was indeed a false theory, he was able to industrialize pasteurization. Through this, bacteria were now able to be removed from the products and it was also able to prolong the products in the dairy industry.

The germ theory of disease has stood as one of the most important theories to be developed during the nineteenth century and proved to shake the science community. It created an everlasting impact that shifted the way scientists saw diseases and how they were caused. This also gave scientists an insight into what was really happening at the microscopic level of a wound that had been infected. A significant change it made in the field of clinical medicine was the sterilization of tools during an operation. Before germ theory, during an operation unsterile equipment had been used to remove a tumor or a bullet that had lodged in the body. This meant that there was a high chance that during the operation, the wound was going to be infected. This just shows one of many impacts germ theory had on the world. Overall, Pasteur was an unreplaceable scientist that took risks, such as his job or reputation, to improve the overall human life. He was a pioneer who sparked the movement on infectious diseases that forever changed the field of microbiology. As a result, Pasteur is remembered for his discoveries in fermentation, pasteurization, and vaccination of rabies.