Germ Theory: Discovering The Little Bugs That Make Us Sick

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It wasn’t too long ago that scientists didn’t even know that many sicknesses were caused by germs. We didn’t have the technology to see them nor the data to show that bacteria and viruses were causing the sickness. And by not too long ago, I mean about a couple hundred years ago.

Today, we’re going to learn that fascinating story behind how scientists discovered that bacteria were actually causing a lot of the sickness in the world and what they proposed we do about it.

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The Story And Science Behind Germ Theory

I don’t know about you, but I was sick multiple times this winter despite thinking my immune system is fairly strong. I got the flu, strep, and a cold all in the span of a couple months. I guess I probably should’ve washed my hands better.

This would kill all of those nasty little germs that were making me sick. Instead, my body had to fight them off. It’s pretty useful to know that washing hands and sanitization are two ways to fight sickness and disease, but often times we take this knowledge for granted.

After all, it wasn’t too long ago that scientists didn’t even know that many sicknesses were caused by germs. We didn’t have the technology to see them nor the data to show that bacteria and viruses were causing the sickness. And by not too long ago, I mean about a couple hundred years ago.

Today, we’re going to learn that fascinating story behind how scientists discovered that bacteria were actually causing a lot of the sickness in the world and what they proposed we do about it.


If you didn’t know about germs, what would you think caused sickness? In the past, many people had their own thoughts. Ideas ranged from bad smells causing people to become ill to sickness being caused by evil spirits.

Because there really wasn’t a known cause for disease along with most of the hypotheses of the time being purely speculative or made up, people got creative when trying to cure sickness. This was especially true in times where there were very few doctors.

Many attempts to cure disease revolved around herbal remedies and astrology. All physicians of the time could really do was take a pulse and listen to the patients’ lungs to make a diagnosis. Like I said, one of the leading theories was that foul smells caused sickness.

This was called the miasma theory, which was popular in the middle ages. To counteract this, some physicians would carry nice smelling flowers or oranges to sweeten the smell which they thought was some sort of poisonous vapor. Aside from having a nice citrusy smell in the air, it’s safe to say that this didn’t do much.

Digging a bit deeper, much of this was centered around the ancient Greek belief of the four humors. They thought that the human body was comprised of four liquids: blood, phlegm, black bile and yellow bile. These were supposed to be in balance for good health and sickness arose when these fell out of balance.

It was also thought that the body tried to balance the humors by excreting excess through urine. Sometimes physicians would try to make a diagnosis by examining urine color, smell, and sometimes taste.

See what a lack of scientific knowledge does? Physicians had to examine urine taste to try and save lives, but the really sad part is that it didn’t really work at all. If it did, would you do it? I think you might be obligated to.

Astrology came into play because it was thought that each humor was associated with a season and also with the position of the moon and stars. Depending on these variables, treatments would vary. They might also perform something called “blood let” where patients would be bled in an attempt to balance the humors.


However crazy these might sound, there is another school of thought that seems equally as crazy to me. It was the philosophy held by the Church which was always suspicious of anything scientific. They believed that illness and the accompanying suffering were simply punishments for sinning against God.

These were some of the main thoughts about illness and treatments of those illnesses or quite some time. Even through the Middle Ages, there wasn’t much change. How would there be? Science wasn’t really progressing and the advancement of thought had gone stagnant.


The Black Plague was really the pinnacle of this twisted way of thinking about disease. The Church used it as an opportunity to get people to go to religious services to pray for forgiveness. Doctors told patients to keep all of their doors and shutters shut and also to light fires to keep the miasma or bad smells away.

Some doctors even told patients not to bathe so that their pores weren’t opened which was thought to let the disease in. Now we know that bathing actually would have been helpful. King Edward III in London ordered that the streets be cleaned of all filth to get rid of foul smells.

Other extremes were seen such as killing cats and dogs in case they were responsible for spreading the disease. Just in case! You can never be too safe I guess.

This may seem quite dismal, and it was. However, a discovery in 1683 changed science and the whole world for the better. Finally, there was a meaningful discovery that could help people and scientists of the time solve these horrendous problems.

A Dutch scientist named Anton van Leeuwenhoek along with an English scientist named Robert Hooke built the first microscope that worked well. It had 200x magnification and with this, he saw some tiny creatures that had never been seen before. He funnily called them animalcules.

While this was a huge discovery, the unfortunate reality of what happens in most of these science stories was shown again. He found these tiny ‘animals’ but there was no context to put it in. Scientists of the time didn’t know what to do with them and didn’t realize the significance of the discovery.


Luckily, in the late 18th and early 19th centuries, a more scientific approach was adopted when approaching medicine. Doctors and scientists started to change their practice based off of tests and observations rather than philosophy and previously held beliefs.

This would all change in 1857 when a French scientist named Luis Pasteur got to work trying to understand the fermentation process grapes go through to make wine. This is a weird place to jump to, but who doesn’t like a little wine, right?

Pasteur used a microscope and was the first to determine that the fermentation process was carried out by yeast. He also found that the process would be ruined if side products were made when the wine got contaminated with other microorganisms.

He was very curios as to where these microorganisms came from. Many scientists at the time thought that they just appeared out of thin air. This was called the spontaneous generation hypothesis. Luis was skeptical, though, and wanted to do more tests before accepting this.

An alternative theory was that microorganisms form from other microorganisms. Pasteur then carried out a set of experiments that resolved this squabble among the scientists. Isn’t it kind of cool how scientists solve arguments?

Instead of getting into a physical fight or having a rap battle, they get to work and do some experiments to prove their point. I guess if they couldn’t solve it this way they might get into a wrestling match or have a rap battle afterwards.


Pasteur was thought to have been partially inspired by the 1858 work of German scientist Rudolf Virchow. He introduced the concept of biogenesis, where he stated that living cells can only exist from preexisting cells.

Following the concept of biogenesis in 1861, Pasteur boiled many different liquids to kill off all of the microorganisms currently living in it and then left it to cool. The design of the flask allowed the liquid to be exposed to air, but was curved down and around so that nothing could float in.

Most of the liquids tested would normally ferment fairly easily, however he found that none of them fermented after he boiled them. He concluded that the reason it didn’t ferment was because microorganisms in the air weren’t allowed to get into the container.

He went further to say that the process of fermentation and decay were caused by microorganisms and they could be killed with heat. This was the birth of pasteurization, the process we still use today with products such as milk, for example.

It’s heated to a point where the bacteria are killed and then it is allowed to cool. His name is literally the process. I think that should be everyone’s goal. To have a famous process named after you. Just take your last name and add ‘ization’ and you’re good to go.


He mostly used this information in the wine industry that he was familiar with. He developed a way to ferment wine without allowing other microorganisms to contaminate it and ruin it. He would heat it to about 50-60 degrees Celsius or about 120-140 degrees Fahrenheit.

Along the way, he also started to characterize several types of bacteria and actually recognized them as different species. He wasn’t the first to discover microorganisms, but he was one of the first to perform rigorous science surrounding them. 


For example, in the late 1700’s the wife of a British ambassador noticed that people with small exposures to smallpox would make them much less likely to get sick. Lady Montague started to facilitate healthy people getting small doses of puss from people with smallpox which was one of the first examples of vaccines being used.

However, this just wasn’t backed by enough evidence to prove that microorganisms cause disease. Then, some Hungarian physicians in the 1840’s noticed that sterilizing their surgery equipment and  washing their hands largely reduced the occurrence of disease in their patients.

The same physician also found that students working with bodies of patients who died from puerpal fever would often somehow pass that to their other live patients. In 1854 the English physician named John Snow realized that the cholera epidemic was coming from a contaminated water fountain and the outbreak stopped after it was shut down.

Little pieces of evidence like this had popped up all over before Pasteur, but none of it had the data that he had. Determining that wine could sort of ‘contract’ disease from microorganisms along with other research showing that microorganisms caused food to spoil was a huge step for determining how people get sick.

He said, "There are similarities between the diseases of animals or man and the diseases of beer and wine." His thoughts were also aided by work he had done on silkworms. He discovered that the silkworms became sick only when they were exposed to other silkworms that were sick.

This meant that there was something being passed from the sick worms to the healthy ones. He then was able to prevent the healthy worms from getting sick by using some sterilization techniques. So, at this point he had pretty much shown that disease was transmitted from sick to the living from the worms and that it could also travel through the air with the yeast.

He called this the “Germ Theory”. The idea was that microorganisms are responsible for much of the disease we see rather than the previous anecdotal hypotheses like the idea that smells or God cause disease.


A few years after Pasteur’s work, German biologist Robert Koch took the ideas and ran with them. The experiment that he did to further determine that microorganisms were responsible for disease was pretty cruel by today’s standards, but his findings may have changed humanity forever.

The deadly bacteria called anthrax had been discovered, but it hadn’t been proven to be deadly yet. However, it was commonly found in sheep that died from disease, so Koch wanted to test it to see if the bacteria was the cause.

He took blood samples from the sick sheep and grew bacteria cultures in his DIY lab at his home. Probably not the safest experiment to run at your house. So, after he isolated the bacteria, he exposed healthy mice, infecting them. His control experiment was to expose healthy mice to blood from healthy sheep.

The mice exposed to anthrax developed the disease and died, while the healthy mice remained healthy. It was a simple experiment, but got right to the point. He showed that it was the bacteria that was responsible for making the mice sick. He published his results and it quickly gained popularity.

Soon after he developed methods for growing pure bacteria strains so that they could be seen clearly under a microscope. I can’t imagine being on of the first to see microbes under a microscope. It’s like looking at an entire world that is all around us, but was never known about. Even today, if you ever have a chance to get under a microscope, try it out.


While this work was monumental for the field of biology as a whole, he developed a universal method for determining whether or not a certain bacteria causes a specific disease which is what he is probably best known for.

His rules for bacteria causing disease are:

1.)The organism must be present in every case of the disease.

2.)The organism must be isolated from a host with the corresponding disease and grown in pure culture.

3.)Samples of the organism removed from the pure culture must cause the corresponding disease when inoculated into a healthy, susceptible laboratory animal.

4.)The organism must be isolated from the inoculated animal and identified as being identical to the original organisms isolated from the initial, diseased host.


These rules were the final dagger to the spontaneous generation theory along with most other theories for disease at the time. However, it took a while for the entire medical community to accept. It helped to usher in what is known to be the ‘Golden Era’ for medical bacteriology.

German biologists around Koch in the next 10 years discovered and isolated the organisms that cause cholera, typhoid fever, diphtheria, pneumonia, tetanus, meningitis, gonorrhea, as well the staphylococcus and streptococcus. This was huge because all of these were silent killers for the time.

At the same moment, a huge movement of sterilization and hygienic practices swept through hospitals, homes and public places in an attempt to slow down these harmful bacteria. Vaccine use also started to really pick up which we will for sure cover in a future episode. Because of all this, who knows how many lives were saved.

In 1905, Koch received the Nobel Prize in physiology and over 21 different disease causing microorganisms had been found. Then, 40 years later, the first antibiotic was discovered which we covered in another podcast. It’s amazing how far humanity has come since the discovery of the germ theory.

In the end, germ theory changed the was doctors, scientists and the general public thought of and reacted to diseases so that we could help prevent them.

I sort of joked earlier about how important science is but think about how humans of the past lived and how different that is from today because of science. In the future, maybe people will look back on us like we look back on the urine tasting physicians of the Middle Ages who thought disease was caused by bad smells and the punishing hand of God.