As clusters of coronavirus infections emerge in the general population and the possibly dozens of new patients from a cruise ship are headed to San Francisco, the Bay Area’s health care facilities are racing to prepare for a sudden surge in demand.

Even in the best of times, hospitals run at near-full capacity. Infection control is a perennial problem. Beds are especially full now because it’s flu season. But coronavirus — infectious, with no vaccine or cure — will test their true readiness.

To save the elderly and medically frail who are at greatest risk of death, hospitals quickly are erecting tents, protecting precious supplies and readying intensive care units and other special services.

For everyone else, they urge: Please stay home.

“In young and healthy people, the disease is mild,” said Dr. Sanjay Kurani, medical director of Santa Clara Valley Medical Center. “Stay home and call your provider.”

Hospitals are protecting stockpiles of critical medical supplies, fearing limits as large distributors suspend shipments. They’re rushing to train staff about the latest guidelines for identifying, isolating and treating patients. And they worry about reduced staffing if workers become sick or must be quarantined.

Meanwhile, they need to make sure they don’t accidentally accelerate the viral spread to other patients. And they must keep providing other essential care, from coronary bypass surgery to knee replacement.

Detailed projections of the expected demand for COVID-19 care have not yet been released by the federal government, but official planning assumptions for pandemic influenza paint a grim picture.

If coronavirus behaves like the 1968 flu, the U.S. Department of Health and Human Services predicts that 38 million people could need medical care and 1 million would be hospitalized. But in a very severe scenario, such as the 1918 flu, the need escalates: 38 million people would still need medical care but 9.6 million would be hospitalized. The U.S. population, by comparison, is 327 million.

There would increased demand for Intensive Care Unit care. In the more moderate scenario, 200,000 would need ICU care; in the severe scenario, 2.9 million would need ICU care. But we currently only have about 46,500 medical ICU beds in the United States, with perhaps an equal number of other ICU beds that could be used in a crisis, write Dr. Eric Toner and Dr. Richard Waldhorn in their February 27 report “What US Hospitals Should Do Now to Prepare for a COVID-19 Pandemic,” published by the Johns Hopkins School of Public Health Center for Health Security.

They conclude: “Even spread out over several months, the mismatch between demand and resources is clear.”

“As there are more and more cases, it is time to start thinking about not how to contain it but how to cope with it — and repurpose our health care facilities to deal with it,” said Dr. Amesh Adalja, an infectious disease specialist at the Center for Health Security.

On Thursday, World Health Organization’s Director-General Tedros Adhanom had stern words for health care leaders:

“This is a time for pulling out all the stops,” he said. “Get your hospitals ready. Ensure essential supplies are available. Train your health workers to identify cases, provide careful and compassionate treatment, and protect themselves from infection.”

To reduce demand, Bay Area hospitals are expanding their telephone and internet-based advice services for those who are otherwise healthy.

At the entrances of emergency departments, they’re rigorously screening patients to identify risk of infection.

To expand the capacity of emergency care, hospitals such as UCSF and Good Samaritan are erecting tents in the parking lots outside entrances.

“They’re military-grade disaster tents,” said Dr. Maria Raven, chief of emergency medicine at UCSF. “We can use them to triage patients that would otherwise be put in a room.”

At Stanford, patients who meet the high-risk criteria — exposure to a known case or recent travel to China, South Korea, Iran and Italy — are directed to a separate entrance for special attention and separation from others.

But as the virus becomes more widespread, these criteria will become less helpful, noted Dr. Yvonne Maldonado, infectious disease specialist and director of Inspection Control at Stanford Children’s Health.

At Stanford’s ambulatory sites, screened patients are placed in a closed-door room and personal protective gear is increased

Every patient who arrives at UCSF with a respiratory illness, whether it is the common cold, flu or coronavirus, gets a mask, said UCSF’s Raven. And they’re moved to a divided or closed room, in isolation.

If a patient is at very high risk, due to known exposure or travel history, they are placed in a dedicated room, and health care workers garb up in gowns, face shields, gloves and masks.

A team of medical specialists — from infectious disease experts to respiratory therapists — then gather to assess the case, said Valley Med’s Kurani.

The severely ill are rushed to special rooms, called negative-pressure isolation rooms — which prevent germs from escaping, and perhaps intensive care.

UCSF has prepared 46 beds in these rooms. If those fill up, one of its new tents could take overflow patients, said Raven.

At Stanford, campus engineers are prepared to create new negative-pressure isolation rooms — even convert an entire ward if necessary.

“They can bring in equipment that sucks the air into the room, and safely vents,” said Maldonado.

Hospitals say that if beds fill up, routine care and elective surgeries can be scaled back. But that’s an economic hardship to the hospitals, because those procedures are the money-makers.

“If we have to start canceling elective surgery, we are prepared to do so,” said Kurani.

As the worried public strips store shelves of masks and other supplies, hospitals are securing their supplies of gowns, gloves and medical face masks, especially so-called N95 respirators.

“We’ve seen some of our stock disappear,” said Maldonado. “We have to keep under lock and key.”

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Watch: Santa Clara County Public Health Department gives updates on reopening schools, Sept. 18 They’re also braced for shortages of healthcare workers — precisely when they need them most. At NorthBay VacaValley Hospital, while caring for a profoundly ill coronavirus patient, at least two health care workers have become infected.

And more than 50 California health care workers have been sent home in recent weeks upon coming into contact with infected patients, according to the Service Employees International Union — United Healthcare Workers West, which represents caregivers.

Five workers at Good Samaritan Hospital in San Jose were sent home Jan. 31 but have since returned to work; at least 10 workers at El Camino Hospital in Mountain View and Los Gatos were taken off the job Feb. 27; two workers at Kaiser Permanente in Los Angeles were quarantined Tuesday; 10 workers at Kaiser Permanente in Roseville were isolated Wednesday; and 30 workers at Sutter Santa Rosa were quarantined Thursday. In Riverside, at least 10 caregivers have been sent home.

“At places like Stanford University Medical Center in Palo Alto, Washington Hospital in Fremont and many Kaiser Permanente facilities across the state, protocols to protect workers, patients and the public have been put in place and are effective,” said Dave Regan, SEIU-UHW president.

Because the full scope of the future outbreak is still unknown, health officials say they don’t yet know if they’ve done too much — or too little.

“You build a fire department ahead of time. You don’t wait for a fire,” said Tom Inglesby of the Johns Hopkins Center for Health Security. “There is an under-appreciation for the amount of time and resources required to build a prepared system.”