Thousands of San Francisco Bay Area residents may have been exposed to measles last week when an unvaccinated student at the University of California, Berkeley, attended classes and rode the area’s BART transit system.

Public health officials in Contra Costa County, outside of San Francisco, said anyone riding BART from Feb. 4 to Feb. 7 during the morning or late evening commutes could have been exposed to the highly contagious respiratory virus. The young man in his 20s lives in the county and was confirmed to have measles on Wednesday. He was likely infected while traveling recently in Asia, health officials said.

Most people in the U.S. are immune to measles, either through vaccination that began in 1963 or because they had the disease. The worry is for unvaccinated people, especially children, who are particularly vulnerable to the disease, which can cause seizures, permanent brain damage or deafness.