Measles is caused by a virus.
A typical case will start with a mild to moderate fever, runny nose, red eyes and a sore throat.
Then other symptoms may start to appear, including a blotchy body rash and tiny white spots in the mouth.
Complications from the disease can lead to ear infections, pneumonia and diarrhea.
It spreads through the air and is highly contagious.
We're talking a reproduction rate of 12 to 18 people for every person infected.
The current vaccine has been used in the United States since 1968.
Before that, almost all children got it before turning 15.
For every 1,000 children who gets measles, one or two will die from it.
Measles can also cause brain damage.
According to the CDC, the best way to avoid getting measles is to be vaccinated.
The measles, mumps and rubella vaccine isn't 100 percent effective, but most of those who caught the measles in a recent U.S. outbreak had not had the shot.
There are currently more than 120 measles cases in 17 states.
The majority of them are linked to an outbreak at California's Disneyland last December.
A handful, like a recent case in Georgia, are not.
Officials say a baby who arrived in Atlanta from outside the U.S. has the measles.
They're trying to keep it from spreading and that's especially hard with measles, because as you heard, it's incredibly contagious.