Public health officials issued a warning Thursday that thousands of Bay Area residents were potentially exposed to measles last week when a UC Berkeley student with the virus attended classes in Berkeley and rode on BART. The student, a Contra Costa County resident whose name was not released, was confirmed to have the highly contagious respiratory disease on Wednesday, and was likely to have been infected while traveling abroad, said officials with Contra Costa Health Services. Health officials urge anyone who shows symptoms such as high fever, runny nose and watery eyes to see their doctor.