This video is supported by Wren, a  website with a monthly subscription that helps fund projects to  combat the climate crisis.

You can click the link in the description to  learn more about making a monthly contribution that supports projects like  rainforest protection programs. [♪ INTRO] Today, we know that infectious diseases  are caused by all kinds of things: viruses, bacteria, fungi, there’s a whole list. That germ theory of disease caught on during  the 19th century, but it wasn’t a smooth road.

One thing standing in its  way was the understanding of how infectious agents could  spread from person to person. It was easier to just… blame bad air. I mean, I don’t blame them for doing  that, that stuff is everywhere.

Still, we got there in the end. And we now recognize that some  of that very important work was done in Cuba in the late 19th century. In the 1880s, yellow fever killed  thousands of people in Cuba.

It’s an awful disease named for the jaundice  experienced by the sickest patients. As their livers fail, a molecule called  bilirubin builds up in their blood, and makes their skin and the  whites of their eyes turn yellow. These patients also experience severe bleeding,  and mortality at that stage is about 50%.

This only happens to a small fraction  of patients… but it’s not very nice. An epidemiologist named Carlos Juan Finlay  initially blamed the disease on miasmas: a theory of infectious disease that  was already on its way out at the time, and attributed their spread to bad, stinky air. But then, Finlay noticed some  things that didn’t line up.

Like, the bleeding he saw in yellow  fever victims looked like it was caused by damage to the cells lining their blood vessels. That wasn’t the kind of damage people  believed a "noxious vapor" could do. So, this led Finlay to a hypothesis.

Because yellow fever damaged  the inside of blood vessels, he thought that whatever was causing the  infection must be in patients’ blood. That meant the disease could only  spread if an infected person's blood somehow ended up in an uninfected person. But it’s not like someone was running around Cuba doing random blood transfusions  on thousands of people.

So, there had to be something  else transferring blood around. A small, buzzing, obnoxious something. Mosquitos.

Finlay noticed that the homes of yellow fever  victims were often full of these insects, and yellow fever season and mosquito season  also seemed to happen at the same time. To learn more, Finlay… well,  honestly, he ran some experiments that we would recognize as super unethical today. Times have changed, and he did take  some measures to protect his patients, which wasn’t a universal practice back then.

But it doesn’t seem great when we look back. In 1881, he had mosquitos bite  a few yellow fever patients. Then, he let the mosquitoes  bite five healthy volunteers.

And behold: One of the volunteers  developed yellow fever nine days later. Fortunately, he recovered. A couple months later, Finlay presented his  findings to the Academy of Sciences in Havana.

But his research didn’t square with how  people understood germ theory at the time. How could it be a mosquito and a germ? So it didn’t catch on right away.

But, Finlay would not be discouraged! To the point where he spend the next nine  years having mosquitos bite healthy volunteers. But this time, his goal wasn't to  prove mosquitos carried the disease.

Finlay believed that a single bite from a  mosquito wouldn’t be enough to make someone sick, but it might be enough to make them immune. We now know his ideas weren’t quite right, but he did manage to hit on  something that would work.. Modern yellow fever vaccines do use a  small dose of the live, yellow fever virus to rile up your immune system.

But vaccines use a weakened form of the virus,  so they rarely have severe side effects. Meanwhile, Finlay's mosquitos were  rolling around with the real deal. So, seven of Finlay’s 102 volunteers got  full-on yellow fever immediately after the bite.

And over the next six years,  another 48 caught the disease. Still, 41 volunteers didn’t get yellow fever  at all during those six years of follow-up. So for as risky as his methods were,  Finlay’s partial success was enough to convince the local Army research team that, hey, mosquitos might actually  transmit yellow fever… even if they weren’t the best vaccination method.

After that, authorities in Havana  implemented… well, martial law. This took place at a time that  the US was occupying Cuba. Measures included water treatment,  and a fumigation campaign to control mosquitoes and eliminate yellow fever.

And it worked beautifully. In less than a year, the disease  had disappeared from Havana. Now, that probably wouldn’t have been possible  without the military controlling civilian life so, this isn’t exactly a great, universal model.

Finlay paints an interesting picture of someone  who initially believed in outdated science, but became convinced by the evidence. And his work no doubt helped advance our  understanding of infectious disease in general. Also, why mosquitoes are truly jerks.

The eradication of Yellow Fever in Cuba was  possible because people listened to science and worked together to make change. And that’s what science policy is all about. Through initiatives like Carbon180  and the Clean Air Task Force, Wren members are supporting policy groups  that advocate for transparent carbon removal and technology to get our  planet to zero-emissions.

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