So far this year, more than 100 people have contracted measles in 21 states, CNN reports. These cases, outlined by the Centers for Disease Control and Prevention in its midyear report, are believed to stem from travelers who carried the disease back to the U.S. from other parts of the world. Most of the affected individuals had not been vaccinated.

Measles is a highly contagious respiratory virus that causes symptoms including a skin rash, fever, coughing, and congestion. Children under the age of five are especially susceptible to the disease and have a higher risk of dying if they contract it.

From January 1 to July 14, 107 cases were reported in Arkansas, California, Connecticut, Florida, Illinois, Indiana, Kansas, Louisiana, Maryland, Michigan, Missouri, Nevada, New Jersey, New York, North Carolina, Oklahoma, Oregon, Pennsylvania, Tennessee, Texas, Washington, and the District of Columbia.

Measles were declared “eliminated” from the U.S. in 2000, meaning that a continuous transmission of the disease did not occur for 12 months in any specific geographic area. However, that doesn’t mean it’s no longer an issue.

In 2014, 667 people contracted measles in the U.S., and 383 of those cases occurred among unvaccinated Amish communities in Ohio. Others were tied to an outbreak in the Philippines.

In recent years, that number has come down, with 86 cases having been reported in 2016 and 118 reported in 2017.

One dose of the measles, mumps, and rubella vaccine is 93 percent effective at preventing measles, while two doses are 97 percent effective. If you believe you've been exposed to measles and aren't sure if you're immune, the CDC recommends calling your doctor to check up on your vaccination record. For unvaccinated patients, a treatment called immune globulin may reduce the risk of contracting measles.

[h/t CNN]