For most of human history, we had a lot of bad ideas
about how we were getting sick ...
angry gods
misaligned planets
stinky fumes.
And plenty of bad ideas about how to prevent it ...
like bloodletting
large holes in the head
arsenic.
Things started changing only about 200 years ago
An English doctor named Edward Jenner decided
to take a closer look at a promising folk remedy.
See milkmaids, they'd get these blisters on their hands - cowpox blisters.
People said that if a milkmaid got cowpox,
she would never get its deadly cousin, smallpox.
So our doctor decided to put this milkmaid wisdom to the test.
His first step: He wiped gunk from cowpox blisters into a cut
in the arm of his gardener’s 8-year-old son, James Phipps.
Sorry James!
James got better pretty quickly 
because cowpox is usually mild in humans.
Then the doctor made his bold and risky move.
He injected James with the similar, but deadly, smallpox.
James didn’t get sick.
The milkmaid cure had worked!
And with that, Jenner invented vaccination.
Here’s how it works:
When you inject weaker germs into our bodies, our immune system
takes care of the problem and stays on the lookout for similar-looking germs.
So when the really bad guys come, we’re ready for ‘em.
This was our first safe and effective way to fight disease.
About time! 
After that, things started happening fast.
Over the next hundred and fifty years,
researchers from all over the world joined this fight against germs.
Discovering them.
Developing vaccines.
Killing bacteria with antiseptics and antibiotics.
So what did that do for us?
Consider that in 1900, the average person lived only about 30 years.
Today, most of us live to 70.
We were so successful in such a short time
in 1967, the U.S. Surgeon General reportedly exclaimed
that we had closed the book on infectious disease.
But the truth … is more complicated.
See, what happened back in episodes one and two -
when humans were cutting down forests and catching new germs from wild animals - 
That’s still happening. 
But now it's on a scale our ancestors never could have imagined. 
We’ve cleared about half of our planet's arable land. 
And we’re still cutting.
Remember, these wild places are where undiscovered germs live. 
By getting rid of their homes, we’re forcing them to find new ones ... near us. 
And there are more of us than ever living closely with animals.
Throw in some planes, and the next thing you know, 
a new germ can turn into a worldwide pandemic practically overnight. 
Think SARS
HIV
Ebola
Bird flu
Zika
These are just some of the viruses plaguing us right now.v
And we haven’t even mentioned  antibiotic resistance. 
So the story continues. Some scientists say the next chapter
is us looking for ways we can live together with germs -
germs in their homes and us in ours. 
Let’s just hope our big brains keep working overtime,
so we find a way to stay one step ahead of that nasty bunch
of fast-changing, human-killing microbes ...
the germs.
