From the Bay Area News Group (via Contra Costa Times):
Public health officials say a person who may be infected with measles visited the West and Central branches of the city library Feb. 27 and 28, potentially exposing hundreds of patrons to the airborne virus.
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Patrons who were at the libraries on the dates should monitor themselves for symptoms until March 21. The virus can linger in the air for two hours after a sick person has left an enclosed space. Symptoms typically develop between seven and 21 days after exposure to the virus.
From the Official Announcement from the City of Berkeley, CA:
On the days in question, the person had not yet developed the tell-tale rash -a circumstance that contributes to the rapid spread of the highly infectious, airborne virus- so did not know that the illness might be measles-. Symptoms can develop between 7 and 21 days after exposure to the virus.
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Berkeley Public Health is working with the libraries to notify and assess individuals who may have been exposed at the facilities. The individual who may have measles is a Contra Costa resident. Berkeley Public Health will work closely with the Contra Costa Public Health Department to follow up, as needed, should measles infection be confirmed.
Read the Complete Announcement