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Podcast Transcript

For thousands of years, many theories have been put forward as to the cause of communicable diseases. 

These theories ranged from the religious to the magical and sometimes quasiscientific, but what they all had in common was that there was no proof for anything. 

Over the centuries these theories became dogma and often prevented a better understanding of diseases. It wasn’t until the 19th century that we got a clear picture of what the cause actually was.

Learn more about the germ theory of disease and why it took so long to recognize on this episode of Everything Everywhere Daily.

Before I begin, I should clarify a few things about the phrase “germ theory of disease” because each of those words can be a bit confusing. 

For starters, “germ” is just a general catch-all term for anything microscopic that can cause disease, including bacteria and viruses. There is no set scientific definition of germ, and the term isn’t usually used in research circles. 

The next is the term theory. A theory does not mean that something is unproven. It is simply a set of statements or principles devised to explain a group of facts or. The theory of gravity is still called a theory even though we have a very good understanding of how gravity works. 

The final word that needs to be clarified is disease. For the purpose of this episode, “disease” will refer specifically to communicable diseases. These are diseases that are spread from one person to another. 

The term “disease” is very broad and is applied to a host of ailments such as heart disease, Parkinson’s Disease, Alzheimer’s disease, alcoholism, and cancer. There are certain ailments that are caused by genetics, diet, exposure to chemicals, and a host of other things that are not germs. 

The term “germ theory of disease” is a historical term that I’m using because that is what it is called, so don’t try to be too pedantic about it. 

With that out of the way, the germ theory of disease is something that I have touched on many times in previous episodes. Ignaz Semmelwize discovered that washing your hands could reduce fatalities in a maternity hospital. John Snow discovered the source of a cholera outbreak in London by tracking where people got their water. 

In a question-and-answer episode, I was asked what one of the most important innovations was that helped make the modern world, and one of my answers was the germ theory of disease. 

So, I figured it was time to devote a full episode to the subject.

The germ theory of disease is something that most people in the world today understand intuitively. We know how to wash our hands, sterilize objects, clean wounds, purify water, and sanitize surfaces where we cook and prepare food. 

The idea is so entrenched that some people develop a psychological condition known as Mysophobia, which is an extreme fear of germs. Something that had never existed before people knew that there were germs. 

Even if you can’t look through a microscope, we can all probably make personal observations regarding cleanliness and health. 

So, why did it take so long to figure this out?

If you remember back to my episode on the Plague of Justinian, epidemics and pandemics didn’t start to occur on a large scale until the development of long-distance trade. 

When such epidemics and pandemics broke out, and people started dying in the hundreds and thousands, no one knew what to make of it. It seemed like a judgment from the gods, which is exactly what most people thought it was.

However, there were some rational people at the time who thought that this wasn’t the doing of the gods. They thought that there was something else going on. 

One of the first people to propose an alternate theory was Hippocrates of Kos, the ancient Greek doctor and philosopher who is considered the father of medicine. 

Hippocrates believed that these diseases were spread because of something in the air. He called this thing in the air miasma. Miasma, he thought, came from rotting organic matter. Miasma comes from the ancient Greek word for pollution.

This was alternatively called bad air, night air, or noxious air at different periods. 

The idea of a miasma wasn’t just a Greek idea. Miasma theory was independently developed in ancient China and India as well.

The Chinese version of miasma actually developed before the Greek version. The Chinese theory was developed in Southern China, where warm, humid air was thought to be a breeding ground for disease. One particular strain of thought believed that insect waste was the source of the poisonous gas and that it was particularly dangerous to go into the deep woods for this reason. 

The Chinese miasma theory developed over time, and many physicians in China thought it was unique to southern China. Some people in Northern China developed a fear of traveling to Southern China because they thought they would become ill from the air. 

In India, a paste that was created from the gambir tree was used as an anti-miasma treatment for diseases. 

I’m actually going to be a little bit sympathetic to ancient people. While they got it wrong, they got it wrong in an understandable way. 

The human nose is attuned to smells of rotting organic matter. We find it offensive. Odors can be experienced from a distance and by many people, and people with diseases often will have odors emanating from them. 

Odors do spread in the air, so at least as an initial hypothesis, the idea wasn’t crazy. The miasma theory held that there were small particles in the air that spread from person to person, and that was how disease spread. 

Part of that was right. However, they just got the mechanism wrong. 

Just as an aside, you can often find a nugget of truth in otherwise wrong theories about nature and science in many ancient authors. They made observations that were true but had wholly wrong explanations for them.

Miasma theory wasn’t the only theory to explain disease and illness. 

In Western medicine, humorism also developed alongside the miasma theory. 

Humorism posited that human health and temperament were governed by four bodily fluids or “humors”: blood, phlegm, yellow bile, and black bile. Each humor corresponded to one of the four elements of nature—air, water, fire, and earth—and was associated with specific personality traits and physical conditions.

Humorism might have developed in Egypt or Mesopotamia, but it became a fully fleshed-out theory in Greece. It also shouldn’t surprise you that it was Hippocrates who also was the one who developed the humorism.

Indian Ayurvedic medicine had a similar system of three humors and five elements. 

These beliefs in miasmas and humors were the predominant beliefs in Western medicine for almost 2000 years. If you learned medicine, which for most of that time was more of an art than a science, you learned about humor and miasmas. 

When the Black Plague struck in the 14th century, many plague doctors wore a creepy-looking mask with a long beak. The reason for the odd-looking mask was miasmas. The beak would often be filled with some sort of pleasant-smelling substance, such as ambergris, mint, or rose petals.

The theory was that if miasma came from foul-smelling rotting substances, then it could be counteracted by pleasurable smells. 

Needless to say, this didn’t really work. 

A version of the germ theory of disease was proposed as early as 1546 by Girolamo Fracastoro. In his seminal work “On Contagion and Contagious Diseases,” published in 1546, Fracastoro posited that diseases were spread by tiny, invisible particles or “seminaria” that could transmit infection by direct or indirect contact, or even over long distances through the air.

This theory was also promoted by the Slovenian physician Marcus Antonius Plencic in 1758. 

However, the theory was not popular and often ridiculed because it went against the established doctrine of miasma theory. 

By the 17th century, optics and lenses had improved enough that it was now possible to see cells and microscopic organisms. In 1665, English scientist Robert Hooke used a microscope to see the first cells and microorganisms. 

Dutch microbiologist Antonie van Leeuwenhoek extended Hooke’s work and the descriptions of microscopic life. 

So, by the start of the 18th century, knowledge of microorganisms was widespread. 

However, even though it was known that they existed, there was still an enormous amount about how they lived and reproduced that was still unknown. Few people thought of linking these tiny creatures to diseases.

One of the problems was yet another erroneous belief in how simple life worked. The prevailing theory was something called spontaneous generation. 

The spontaneous generation held that life could arise from nonliving matter and that it happened all the time. 

An example of this would be if you left a piece of meat out, and after a couple of days, maggots would appear on the meat. According to the theory of spontaneous generation, the meat created the maggots. 

Spontaneous generation goes back just as far, if not further, than the theory of miasma, and it was coherently synthesized by Aristotle. 

So when microscopic life was found, the framework it fit into was one where such life forms could just appear from the material surrounding it. The idea of how it could reproduce and spread wasn’t something that was immediately considered. 

It wasn’t until the 1830s that Matthias Schleiden and Theodor Schwann developed the theory that all living things were made up of cells. This theory was crucial because it suggested that diseases might involve cellular changes, possibly due to external agents.

In 1865, the Hungarian physician Ignaz Semmelwize, to whom I’ve previously devoted an entire episode, realized that something was spreading from corpses to mothers giving birth at a maternity hospital. 

If you remember back to the episode, his proposal for washing hands to stop the spread of disease wasn’t just rejected, it was vehemently rejected by the medical establishment. 

The person who really changed medical orthodoxy and put a nail into the theories of miasma, humorism, and spontaneous generation was Louis Pasteur. 

Pasteur’s experiments in the 1860s refuted the notion of spontaneous generation, showing that organisms come from other organisms and do not spontaneously appear. This was pivotal in suggesting a biological basis for the transmission of disease.

The other person who played a pivotal role was the German microbiologist Robert Koch, who developed a series of criteria, dubbed Koch’s postulates, to demonstrate the causal relationship between a microbe and a disease. These postulates became the gold standard for identifying the microbial causes of diseases.

The postulates are:

1. The microorganism must be found in abundance in all organisms suffering from the disease but should not be found in healthy organisms.
2. The microorganism must be isolated from a diseased organism and grown in pure culture.
3. The cultured microorganism should cause disease when introduced into a healthy organism.
4. The microorganism must be re-isolated from the inoculated, diseased experimental host and identified as being identical to the original specific causative agent.

These postulates provided a way to prove that a given microorganism caused a disease. It should be noted that he modified the first postulate when he discovered asymptomatic carriers of some diseases.

Koch applied these postulates himself to identify the pathogens that caused ??cholera and tuberculosis. 

Once the work of Koch and Pasteur gained acceptance in the late 19th century, the race was on and there were rapid advances in immunology, public health, and the development of medical microbiology as a science.

There was now a theory that could explain why the practice of inoculation worked to stop smallpox because the germ theory explained what caused smallpox. 

Clean water, sterilizing surgical instruments, vaccines, antiseptic bandages, antibiotics, everything antibacterial,  and a host of other innovations that improved life expectancy in the 20th century were all the result of the germ theory of disease. 

As I noted at the start of the episode, the germ theory of disease hasn’t solved every health problem, and antibacterial resistance, which is an overapplication of the germ theory, has caused a host of problems all its own. 

But many illnesses, such as smallpox and cholera, which once ravaged humanity, have been all but eliminated. It was all due to abandoning the pseudoscience of miasma and accepting the reality of germs.