The number of cases of measles in the United States is now at its highest level in 25 years.

Here's what we know

There have been 839 cases in 23 states so far this year. The number of cases hasn't been this high since 1994. There were 75 cases last week alone. The Centers for Disease Control and Prevention blames this rise on people opting out of vaccinations for themselves and their children.

Before the measles vaccine was developed in 1963, 400 to 500 people would die from it each year, and 48,000 would be hospitalized. The disease can still be fatal, particularly in small children.

In 2000, the disease was thought to have been entirely eradicated in the United States.

In an April 25 news release, the CDC blamed the recent outbreak on "an unvaccinated traveler" — either someone from another country, or an unvaccinated American who traveled out of the country and then returned home — who entered the United States with the virus.

The agency explained, "When measles is imported into a community with a highly vaccinated population, outbreaks either don't happen or are usually small. However, once measles is in an under-vaccinated community, it becomes difficult to control the spread of the disease."

What else?

Most of the confirmed cases of measles in 2019 occurred in people who were not vaccinated. There's a small chance of getting the disease even after being vaccinated (the vaccine is 97 percent effective), but the odds of these people being infected goes down if most of the population is immune.

The CDC also said that most of the people who are vaccinated but still for some reason susceptible to the virus get a less extreme case than they would have if they were unvaccinated.

As the number of cases continues to increase, two universities in California have gone on quarantine to prevent the spread of the disease, and officials in Rockland County, New York, have declared a state of emergency.