An inmate at San Quentin State Prison on San Francisco Bay was diagnosed with Legionnaires' disease and about 20 others showed symptoms of pneumonia, prompting officials to shut off water service at the institution, a prison spokeswoman said on Friday.

Legionnaires' disease, a severe form of pneumonia, is caused by inhaling mist infected with the bacteria Legionella. Symptoms include fever, cough, chills and muscle aches.

The diagnosed inmate, along with the others exhibiting symptoms, are all being treated at an outside hospital, said Dana Simas, a spokeswoman for the California Department of Corrections & Rehabilitation.

None of the inmates are in grave condition, she said.

Public health officials ordered the prison to cease all water activities until the source of the exposure could be determined, Simas said. In the meantime, the prison is bringing in water from secondary sources, she said, including bottled water and portable toilets.

Earlier this month officials in New York City announced the end to a Legionnaires' outbreak, which killed 12 and sickened 128 people there.