Officials at San Quentin State Prison say the prison is on a medical lockdown after at least two inmates became sick with chickenpox.
Prison spokesman Lt. Sam Robinson says the prison has been locked down since last Friday, with only employees being allowed to enter and leave the facility.
He could not say when the lockdown would be lifted.
Robinson says the last time there was a medical lockdown at the prison was in March of last year when at least four inmates became sick with chickenpox.
The Centers for Disease Control describes chickenpox as a "very contagious disease" that spreads easily from infected people to others who have never had chickenpox or have never received the chickenpox vaccine.
San Quentin, located north of San Francisco, is the oldest prison in California. It houses about 3,800 inmates.
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