Students who spent time at Washington Dulles International Airport earlier this week may be at risk of exposure to measles.

Alison Ansher, the health director of Prince William County’s Health District, told WAMU Thursday that a child who contracted measles from traveling abroad may have infected travelers at Dulles Airport Sunday. Officials said they are working to identify anyone who might have been exposed to the virus at Dulles or at two other locations, Novant Health UVA Health System Haymarket Medical Center and Inova Fair Oaks Hospital, WAMU reported.

Ansher said travelers at the airport’s Terminal A and at the baggage claim level Sunday from 5:30 to 8:30 p.m. could be at risk of exposure to measles, WAMU reported. Visitors and patients at Haymarket Medical Center from Sunday at 11 p.m. to Monday at 4:30 a.m. and at Inova Fair Oaks Hospital Tuesday from 3 to 5:30 p.m. should also be aware of potential infection.

“If an individual feels that they were exposed because they were at one of the three sites at the specific time spans, we are calling them, or they are calling us,” she told WAMU.

Ansher added that people who believe they might be infected should reach out to their local health department or doctor to learn more about minimizing the risk of infecting others, WAMU reported. She said those who believe they are infected should notify their doctor’s office or emergency room about their condition ahead of arriving, according to WAMU.

Measles is a highly infectious disease that can trigger pneumonia and encephalitis, or brain swelling, in patients, according to the Centers for Disease Control and Prevention. These conditions could cause death, deafness or the development of an intellectual disability, especially in children.

There have been more than 1,000 cases of measles in the United States since January, according to the CDC. Health and Human Services Secretary Alex Azar released a statement Wednesday urging people to receive their vaccines to protect themselves and prevent the continued spread of measles.

“We cannot say this enough: Vaccines are a safe and highly effective public health tool that can prevent this disease and end the current outbreak,” he said in the release. “The measles vaccine is among the most-studied medical products we have and is given safely to millions of children and adults each year.”