San Francisco health officials are warning that thousands of people may have been exposed to measles last week.
They say a U.C. Berkeley student with a confirmed case of measles attended classes at Berkeley and also rode the BART mass transit system.
Anyone who also used BART between Feb. 4 and Feb. 7 during the morning commute or late evening hours may have been exposed to the highly contagious virus, which spreads through coughs and sneezes.
"Most people are immune, but we really want to make sure that if people have these symptoms that they do let their health care provider know it could be measles," Erika Jenssen, the communicable disease program chief of Contra Costa County, said.
People who have never been vaccinated against measles are most at risk.
Anyone who hasn't been vaccinated or who develops symptoms like high fever and a runny nose is urged to see a doctor.