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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.
BART transports nearly 400,000 riders each weekday.
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.
"We're definitely concerned about people who are not immune," said Erika Jenssen, a spokeswoman for Contral Costa Health Services.
Home-grown cases of measles were considered eradicated in the U.S. in 2000, but the nation has seen a recent uptick in outbreaks because of imported infections brought back by travelers from areas where the disease remains common. The infection can spread rapidly where there are pockets of unvaccinated people. The Bay Area is considered one of the prime sites for unvaccinated children in the U.S., experts say.
Usually there are fewer than 60 cases per year, but in 2013, at least 175 cases were reported in the U.S., according to the Centers for Disease Control and Prevention. At its peak, measles killed 500 people a year in America and hospitalized about 48,000.
Health officials are reviewing the student's movements and notifying people who were in close contact with him. So far, no other measles cases have been identified in connection with this case.
People who may have been exposed should look for symptoms such as high fever, runny nose and watery eyes, as well as the characteristic red rash. Anyone who appears to be ill should seek medical care.