For some members of an Avengers: Endgame audience in Fullerton, California last week, the danger wasn’t limited to what was happening onscreen. According to a report in the Los Angeles Times, a woman recently infected with measles—but without measles symptoms—may have exposed the crowd to the virus.

Health officials in Orange County announced that the woman, who lives in Placentia, had contracted the disease while traveling overseas. Upon returning to California, she went to work and attended a midnight movie—likely a screening of Avengers: Endgame Thursday evening—before coming down with measles symptoms. As a result, officials are warning anyone who was at the AMC Theaters location on 1001 South Lemon Street in Fullerton from 11 p.m. April 25 to 4 a.m. April 26 to review their measles vaccine history and be mindful of possible symptoms, such as a measles rash. People in the vicinity of her workplace and the emergency department in Fullerton she visited are also being cautioned.

Measles causes fever, rash, coughing, and watery eyes, and can progress to more serious illnesses like pneumonia and pregnancy complications. The virus is transmitted through the air via coughing or sneezing and can linger for up to two hours.

This isn't California's first brush with the disease. Universities in the Los Angeles area have experienced an outbreak, with students and staff members told to remain home until their immunization records can be evaluated.

According to the Centers for Disease Control and Prevention, 704 people have been diagnosed with measles nationwide this year, the worst year for the disease since 1994.

In 1989, the CDC recommended two doses of the MMR vaccine—which protects against measles, mumps, and rubella—when a child is between 12 and 15 months old and again between 4 to 6 years old. The vaccine is thought to provide a 97 percent chance of protection against measles. Prior to that, a single-dose vaccine was typical but it offered less protection. Adults born before 1957 were likely exposed to the virus as children and are considered immunized. People who were not vaccinated as a child, unsure of their status, or at high risk due to working in health care or traveling internationally can still receive an MMR shot. They should check with their physician.

[h/t Los Angeles Times]