We’re pretty solid on germ theory at this point.

The idea that many diseases are caused and spread by pathogens too small to seen has been locked in since the mid 19th century. But that wasn’t the first time  people came up with those ideas.

Physicians around the world had been theorizing about the spread of infectious  disease through invisible particles as early as the 9th century. That’s one thousand years earlier. These scholars did everything they  could to limit the spread of infection and understand how it worked.

But if we had the basis for germ theory … by all accounts a correct idea … why did it take so long for  it to actually take root? Well, because right ideas don’t  always win – not right away. And sometimes wrong ideas  can be pretty convincing.

So let’s take a look at why one of  the most important ideas in biology took centuries to accept. [intro music] Before germ theory, the main idea behind the spread of  diseases was called miasma theory. Miasma theory says that you  catch diseases from bad air, so anything that smelled bad or  rotten was the reason you’d get sick. In some ways, it makes sense  why people believed this.

Places that are crowded and smelly do  also tend to be hotbeds of disease. Although we now know it’s because  of the viruses and bacteria that come with waste and crowded  areas, not the air itself. It just so happened that many  of the prevention methods against miasma also prevented the transmission  of disease from germs.

For example, people got rid of “bad air” by swiftly removing waste and  dead bodies from populated areas, which is still an important way  to prevent disease transmission in the modern era. But around the time of the  Black Death in the 1300s, it became clear that miasma theory was not  enough to explain the spread of disease. Some people looked for explanations  to augment miasma theory, including that the celestial  energies of Mars and Jupiter were drawing up toxic air from the earth.

And you thought Mercury  being in retrograde was bad… But some people had another idea. They started thinking that diseases might actually come from  something other than the air: a contagion. Some European cities started implementing  quarantines on incoming ships to make sure that sailors weren’t  bringing in any diseases with them.

That’s not to say that everyone  was suddenly Team Contagion now. But as the Black Death  continued to devastate Eurasia, people began to put more contagion-based  ideas together with miasma. Some doctors thought that while the body could make more of a disease  and spread it to others, miasmas were what caused the initial illness.

In fact, the classic plague doctor  mask was born from miasma theories during the Black Death. The iconic beak shape was created  to store sweet-smelling things for the doctors to breathe  instead of the stinky, sick air, and it included spices like cinnamon and myrrh. Oh, and ground up vipers.

Just for funsies. This wasn’t even the first time in history that people were combining  miasma and contagion theories. In the ninth century, a  scholar named Qustā ibn Lūqā hypothesized that some diseases  were spread by foul air, and others by a spark of illness that can  jump from a sick person to a healthy one.

In 1546, an Italian physician  named Girolamo Fracastoro published the first known work proposing  a fully contagion-based theory of disease. Despite having no knowledge of microbes, he theorized that diseases were  spread by so-called “seeds” that caused illness when they  came into contact with people. Fracastoro started his disease studies by  looking at syphilis and animal outbreaks.

He noticed that syphilis seemed to pass primarily from person  to person through sexual contact, and sick animals would get the entire herd sick if they weren’t isolated. After reading a poem that described  invisible seeds floating through the air, some fostering life and others creating disease, Fracastoro found his analogy for contagion. Never let anyone tell you that  scientists don’t need to study the arts.

Fracastoro hypothesized that seeds  of disease were all around us, and they could multiply inside a host  before spreading to other people. They could spread through  direct contact, like syphilis, they could float through the air, or they could be passed from person to person through a contaminated object. He coined the term fomites for these surfaces that can pass on disease from one person to  another, and we still use that term today.

Even though Fracastoro got  many things shockingly spot on, his ideas were mostly passed over for 300 years. In good scientific fashion, he presented his thoughts on  seeds and contagion to his peers, who debated and ultimately rejected it. And it’s not that his colleagues  were being unscientific by rejecting his ideas.

At this point, Fracastoro  was simply hypothesizing, and he didn’t do any experiments  to back up his claims. It’s funny how things that  we may take for granted now, seemed like wild, half-baked  ideas back in the day. I mean, really.

Imagine our world full of tiny little dudes, completely self sufficient and self  replicating and living complex lives, then wiping out the human population on the side. Oh, and they’re invisible by the way. Come on!

Why would you think that when stinky air is literally right there under your nose? Science needed the proof of actually seeing a whole hidden world of microorganisms in order to accept Fracastoro’s  ideas for what they were. It’s not really Fracastoro’s fault.

He couldn’t do the experiments  that his theory so badly needed. It wouldn’t be until decades later  that the first microscope was invented, then another hundred-ish years  for Antonie van Leeuwenhoek to observe bacteria for the first time. Even as more and more people started  to buy into contagion theories, it would still take more convincing for scientists to fully hop on the contagion bandwagon.

Around the 18th century, people started to split into  two camps of disease theory, and like all good rivalries, they got team names. You have contagionists, who were backing the idea that  disease is caused by microorganisms, and you have miasmatists, who were backing the idea of, well, miasma. Since miasma was already the dominant worldview, contagionists had the burden of proof.

A lot of early contagionist ideas were able to be explained away by miasma, and coincidences, like patients surviving ineffective  miasma-based treatments, fed into the preexisting beliefs. Contagionists needed clear proof  to sway the scientific community, and throughout all this time, they still didn’t have anything to point to as a clear cause-and-effect  from a germ to a disease. The proof took all the way until the late 1800s, when Robert Koch isolated anthrax  bacteria from cows to infect mice, who then got anthrax and died.

This experiment confirmed what Fracastoro and so many others had suspected all along: germs cause disease, not bad air. From there, germ theory really took flight, and a thorough series of  experiments during the early 1900s finally settled the debate. Even if Fracastoro was just  spitballing at the time, he was vindicated eventually.

Science is a process, and sometimes that process… takes a while. But it’s a process that  helps ideas rise to the top when they’re supported by evidence. And thanks to germ theory, we have vaccines and huge advances  in public health… finally.

This video was inspired by  Air-borne, a new book by Carl Zimmer. It’s a history of airborne disease  and how it has shaped the world. If you’d like to check it out,  head to the SciShow Bookshop page.

This video was made possible by the Alfred P. Sloan Foundation. [ OUTRO ]