A student from the University of California, Berkeley rode San Francisco’s Bay Area Rapid Transit (BART) system while infected with measles. Public health officials fear thousands could have been exposed to the disease.

According to SanJoseMercuryNews health officials have confirmed that the student had not been vaccinated from the disease. It is probable that he was infected during a recent trip to Asia.

According to cbsnews Erika Jenssen of Contra Costa County Public Health said, “If this individual had been vaccinated and was exposed to measles abroad, they wouldn’t have been infected and would not have brought the disease to the United States.”

The disease is highly contagious and spreads through droplets passed through the air by cough or sneeze. Symptoms include a blotchy rash, fever, and white spots inside the mouth. The rash generally starts on the face and spreads to the body.

Authorities are warning passengers who rode BART anytime between February 4 and February 7 to seek medical attention if any symptoms appear.