Here in the US, a disease that was all but extinct has made a comeback.
Here in the US, a disease that was all but extinct has made a comeback.

I’m talking about measles.
I’m talking about measles.

It usually starts with flu-like symptoms and a raised rash on the face, followed by inflammation
It usually starts with flu-like symptoms and a raised rash on the face, followed by inflammation

of the eyes, and then fevers as high as 40.5 degrees Celsius -- that’s 105 Fahrenheit
of the eyes, and then fevers as high as 40.5 degrees Celsius -- that’s 105 Fahrenheit

-- and in the very young, brain damage, hearing loss, and even death.
-- and in the very young, brain damage, hearing loss, and even death.

The disease has been so rare in the U.S. that it was pretty easy for scientists to figure
The disease has been so rare in the U.S. that it was pretty easy for scientists to figure

out where the outbreak started.
out where the outbreak started.

In December, a person carrying the measles virus visited Disneyland in California.
In December, a person carrying the measles virus visited Disneyland in California.

Now, more than 100 people have been infected, and it has spread to seventeen states.
Now, more than 100 people have been infected, and it has spread to seventeen states.

Now, yeah, 100 people might not sound like a lot.
Now, yeah, 100 people might not sound like a lot.

But keep in mind that that is twice the number of cases the US usually sees in an entire
But keep in mind that that is twice the number of cases the US usually sees in an entire

year, and that the outbreak is only two months old.
year, and that the outbreak is only two months old.

Now the Centers for Disease Control is concerned that the outbreak could gain a foothold unless
Now the Centers for Disease Control is concerned that the outbreak could gain a foothold unless

more people are vaccinated.
more people are vaccinated.

So how does something like this happen?
So how does something like this happen?

I’m Hank Green and this is SciShow News.
I’m Hank Green and this is SciShow News.

[Intro]
[Intro]

In the year 2000, scientists declared that measles had been officially eliminated from
In the year 2000, scientists declared that measles had been officially eliminated from

the United States.
the United States.

Elimination, in a medical sense, means that a disease is no longer endemic, or regularly
Elimination, in a medical sense, means that a disease is no longer endemic, or regularly

found in a certain region.
found in a certain region.

But eliminating the disease in the U.S. didn’t change the fact that it was still prevalent
But eliminating the disease in the U.S. didn’t change the fact that it was still prevalent

in other countries, like the Philippines -- where the current strain of measles seems to be
in other countries, like the Philippines -- where the current strain of measles seems to be

from -- and it remains a leading cause of death among young children worldwide.
from -- and it remains a leading cause of death among young children worldwide.

Though, thanks to vaccinations, worldwide Measles deaths have decreased 78% in the last
Though, thanks to vaccinations, worldwide Measles deaths have decreased 78% in the last

ten years.
ten years.

Now, measles is a viral disease that’s communicable through airborne droplets of water, just like
Now, measles is a viral disease that’s communicable through airborne droplets of water, just like

flu, and the viruses can linger on surfaces for a couple hours.
flu, and the viruses can linger on surfaces for a couple hours.

One infected person will infect, on average, 12 to 18 other people.
One infected person will infect, on average, 12 to 18 other people.

That’s one of the reasons why this outbreak has spread so rapidly.
That’s one of the reasons why this outbreak has spread so rapidly.

But of course, there is a vaccine for measles -- it’s been around for 50 years.
But of course, there is a vaccine for measles -- it’s been around for 50 years.

It uses a small amount of live measles virus that’s been weakened by a process called
It uses a small amount of live measles virus that’s been weakened by a process called

cell culture adaptation.
cell culture adaptation.

This process basically genetically modifies the virus by allowing it to grow inside of
This process basically genetically modifies the virus by allowing it to grow inside of

some kind of medium that isn’t human cells.
some kind of medium that isn’t human cells.

In the case of the measles vaccine we do this basically with, like, chicken embryos in a
In the case of the measles vaccine we do this basically with, like, chicken embryos in a

petri dish.
petri dish.

After a while, the virus adapts to infecting chickens, and these adaptations make it less
After a while, the virus adapts to infecting chickens, and these adaptations make it less

virulent to humans.
virulent to humans.

While a natural measles virus reproduces thousands of times during an infection, a weakened virus
While a natural measles virus reproduces thousands of times during an infection, a weakened virus

can only reproduce about twenty times inside of a human host.
can only reproduce about twenty times inside of a human host.

But that’s just enough so that it doesn't cause any symptoms, but it does stimulate
But that’s just enough so that it doesn't cause any symptoms, but it does stimulate

an immune response in our bodies, which allows us to produce antibodies.
an immune response in our bodies, which allows us to produce antibodies.

Throughout the rest of our lives, those antibodies will be able to identify a measles virus so
Throughout the rest of our lives, those antibodies will be able to identify a measles virus so

that our immune system can be awesome at destroying it.
that our immune system can be awesome at destroying it.

But … the system isn’t quite perfect.
But … the system isn’t quite perfect.

3 to 5 percent of the population can still contract measles even if they’re vaccinated.
3 to 5 percent of the population can still contract measles even if they’re vaccinated.

And yet, if a high enough percentage of the population is immunized, the virus can’t
And yet, if a high enough percentage of the population is immunized, the virus can’t

spread.
spread.

Instead of each infected person creating twelve or eighteen new disease vectors, the average
Instead of each infected person creating twelve or eighteen new disease vectors, the average

person creates less than one new disease vector, and thus, the outbreak just doesn't happen.
person creates less than one new disease vector, and thus, the outbreak just doesn't happen.

This, my fellow herd-members, is called herd immunity.
This, my fellow herd-members, is called herd immunity.

And it means that an entire community can/be protected, even if a few of those members
And it means that an entire community can/be protected, even if a few of those members

aren’t immune to the pathogen.
aren’t immune to the pathogen.

In the case of the measles, 92 to 94 percent of a population has to be immunized in order
In the case of the measles, 92 to 94 percent of a population has to be immunized in order

for herd immunity to protect the whole group.
for herd immunity to protect the whole group.

So, how do we know all this?
So, how do we know all this?

Well, a widespread vaccination program for measles, mumps and rubella -- all airborne
Well, a widespread vaccination program for measles, mumps and rubella -- all airborne

viral diseases -- began in the US in 1963.
viral diseases -- began in the US in 1963.

And as the country’s herd immunity increased, the incidence of measles went from 3 to 4
And as the country’s herd immunity increased, the incidence of measles went from 3 to 4

million cases a year in the 1950s down to just 37 cases in 2004.
million cases a year in the 1950s down to just 37 cases in 2004.

But in recent years, immunization rates have began declining, due in part to the growing
But in recent years, immunization rates have began declining, due in part to the growing

misconceptions that have erroneously linked these vaccines to autism and other disorders.
misconceptions that have erroneously linked these vaccines to autism and other disorders.

As a result, in nearly 500 public schools at ground zero for the current outbreak -- in
As a result, in nearly 500 public schools at ground zero for the current outbreak -- in

Southern California -- the immunization rate has fallen below that 92 percent threshold,
Southern California -- the immunization rate has fallen below that 92 percent threshold,

which has given the measles an opportunity to spread again.
which has given the measles an opportunity to spread again.

But people who’ve contracted measles from this latest outbreak are all over the country
But people who’ve contracted measles from this latest outbreak are all over the country

-- more than half of them weren’t vaccinated.
-- more than half of them weren’t vaccinated.

Immunization rates in eight other states are even lower than California’s -- with Colorado
Immunization rates in eight other states are even lower than California’s -- with Colorado

coming in last, at around 81 percent.
coming in last, at around 81 percent.

So this is probably not the last we’ve heard from measles.
So this is probably not the last we’ve heard from measles.

Our herd is being compromised.
Our herd is being compromised.

But, how did we get here?
But, how did we get here?

How did we get to the point where people stopped using safe and effective protection against
How did we get to the point where people stopped using safe and effective protection against

a disease, and decide instead to take their chances?
a disease, and decide instead to take their chances?

We’ll be talking about the science of why we sometimes ignore science, in a special
We’ll be talking about the science of why we sometimes ignore science, in a special

infusion next week, so be sure to join us.
infusion next week, so be sure to join us.

In the meantime, thanks for watching SciShow News.
In the meantime, thanks for watching SciShow News.

If you want to help us share science with the world, you can become a supporting subscriber
If you want to help us share science with the world, you can become a supporting subscriber

at Subbable.com/scishow.
at Subbable.com/scishow.

And if you want to keep getting smarter with us, you can go to youtube.com/scishow and
And if you want to keep getting smarter with us, you can go to youtube.com/scishow and

subscribe.
subscribe.