Visitors and new inmates are being barred at the Chino women's prison because of a suspected gastrointestinal illness sweeping the San Bernardino County facility.
Eighteen people at the California Institution for Women had come down Friday with symptoms of the contagious intestinal illness, including diarrhea and vomiting.
Prison spokesman Robert Patterson said Monday there is one confirmed case of gastroenteritis, an inflammation of the large and small intestines, and 17 others have symptoms.
The prison population can reach 2,400 inmates.
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As a precaution, the prison has temporarily suspended weekend family visits, parole hearings and inmate transfers. The general population, psychiatric services unit and administrative segregation are quarantined.
Common areas, including dining halls, are being sanitized with bleach.