The California Department of Corrections and Rehabilitation, which runs the well-known prison, announced Friday that the water in the facility has been shut off after an inmate was diagnosed on Thursday with Legionnaires' disease. He was in stable condition Friday evening.

The number of cases may grow, with another inmate hospitalized with symptoms of the disease and about 30 more inmates "under observation for pneumonia-like symptoms."

San Quentin is one of the United States' most famous prisons, not just because of singer Johnny Cash's 1958 visit there but because of the hard-core criminals who have called it home. The prison houses about 3,700 inmates, which is several hundred over its designed capacity, including about 700 on death row, according to the CDCR.

While it's not known if more than one current inmate has Legionnaires', many more inmates have been affected and in a lot of ways.

The state agency initially announced Friday that all water had been shut off in the complex "until the cause of the exposure is discovered."

As a result, water and porta-potties were brought in for inmates' use. And showering or "any other activities that create steam, such as kitchen operations" were prohibited, per orders from the Marin County Public Health Department.

The CDCR amended its policy in a new press release later Friday, saying water use was still being limited albeit not to the same extent as before.

Inmates or San Quentin's roughly 1,800 employees still won't drink water out of taps. But regular, plumbed toilets were once again OK to use in housing units, as was "monitored use of water for cooking," according to the state corrections agency.

On heels of deadly Legionnaires' outbreak in New York

The reason prison and health authorities focused on steam and mist is because those are two of the main ways people are exposed to the Legionella bacteria, thus the corresponding disease, according to the Centers for Disease Control and Prevention.

While it got its name from a 1976 outbreak at a Philadelphia convention for the American Legion, Legionnaires' is hardly a new disease. Nor is it particularly rare, with the CDC estimating 8,000 to 18,000 hospitalized cases each year in the United States.

It doesn't spread from person-to-person but rather through the air, with those getting it typically coming down with a fever, chills and a cough. Most recover, but 5% to 30% of those who get the disease die, according to the CDC.

This summer, 12 people in the South Bronx died and more than 115 people were hospitalized after contracting Legionnaires' disease, according to the New York City heath department. All the dead were adults with underlying medical conditions.

Hotel's cooling tower ID'd as source of outbreak

Laboratory tests subsequently traced the outbreak to bacteria found in a cooling tower in the Opera House Hotel that matched the strain found in patients.

An outbreak of the disease killed two people at a hotel in downtown Chicago in 2012.

CNN's Amanda Watts contributed to this report.