At least six San Quentin State Prison inmates were ill with Legionnaires' disease and dozens more under observation Sunday, prompting a weekend halt to visitors, no hot meals and limited drinking water supplies at California's oldest prison.

At least 51 inmates are under observation for respiratory illness at the prison's medical unit, said Dana Simas, a spokeswoman with the California Department of Corrections and Rehabilitation.

Cooking at the prison has stopped because Legionella bacteria grow in water and spread through water molecules. Instead inmates are being served boxed meals, Simas said.

Water use at California's oldest prison has been limited since last week, when an inmate was hospitalized with a confirmed case of the disease. Officials brought in portable toilets, bottled water and large water tanks to serve thousands of inmates and employees.

The disease, caused by a bacterium, is considered a severe type of pneumonia that can bring high fever, chills and a cough. It occurs when contaminated water is inhaled into the lungs in the form of steam, mist or moisture. It is considered particularly dangerous for older people and those with underlying health issues.

Water from the prison's plumbing supply was being used only for inmate toilets and for cooking. Portable showers for the 3,700 prisoners were brought in Saturday.

Meanwhile, thousands of gallons and liter bottles of water have been hauled in to the 163-year-old prison north of San Francisco.

The prison was closed to visitors and volunteers through the weekend, though officials said the public was not believed to be in danger. None of the prison's more than 1,200 employees have been sickened.

A recent outbreak that sickened 128 people and killed 12 in New York City was traced by the city's health commissioner to a rooftop air conditioning unit at a Bronx hotel.

Once officials identify the source, they generally use higher-than-normal levels of chlorine to kill the bacteria. Water at the prison usually comes from a tank that can hold about 3 million gallons.