San Francisco Bay Area officials say a University of California, Berkeley, student infected with measles could have exposed thousands of others by attending classes and riding public transit.
Public health officials said Thursday they confirmed that the student in his 20s was not vaccinated, and was likely infected with measles during a recent trip abroad.
The student attended classes in Berkeley and took BART trains last week, possibly exposing thousands before his diagnosis.
Health officials say people who have had measles before, or who are vaccinated, are unlikely to be infected even if they have contact with the contagious person.
Those without a vaccination are at risk of catching the disease if exposed to the virus.
Officials say those showing measles symptoms should contact their health care provider immediately.