LONDON — The shaky cellphone video pans across the cavernous convention center that is being converted into three vast coronavirus wards. The space goes on and on — six football fields long. It had been booked for a wedding expo this week, before the ban on large gatherings. Instead, it will house as many as 4,000 beds and two morgues.

“If you’re not taking it seriously, like I wasn’t, I think we really need to start, because they’re preparing for an absolute high death toll here,” said Alex Woodside, who was laying cables at the temporary Nightingale Hospital in east London’s ExCel Centre when he took the video, which he posted on Facebook.

British newspapers called the footage “chilling.”

Footage shows London's ExCel Centre being converted into the Nightingale Hospital on March 29 to cope with an expected peak in coronavirus cases in the capital. (Press Association (PA))

Just as foreboding, Britain’s National Health Service announced this week that the need for staff members at the Nightingale is so great that furloughed flight attendants from easyJet and Virgin Atlantic airlines will be sent in to assist doctors and nurses. The cabin crews will perform duties such as changing bedding.

That countries throughout Europe and some cities in the United States are scrambling to outfit what are essentially field hospitals, in spaces that recently held comic book conventions and dog shows, illustrates how massive the coming wave of coronavirus patients may be, and how underprepared existing health-care systems are.

In Britain, Prime Minister Boris Johnson, Health Secretary Matt Hancock, England’s chief medical officer, Chris Whitty, and Brexit mastermind Dominic Cummings, Johnson’s top aide, are all sick with the coronavirus and self-isolating.

“If we can keep deaths below 20,000, we will have done very well in this epidemic,” said Stephen Powis, Britain’s national medical director.

Deborah Birx, coordinator for the White House's coronavirus task force, told NBC News on Monday, “If we do things together well, almost perfectly, we could get in the range of 100,000 to 200,000 fatalities.”

Public health experts say temporary hospitals such as the Nightingale in London, which should be ready to admit its first 500 patients this week — or the Jacob K. Javits Convention Center in Manhattan, where 1,000 beds are waiting — can provide an adequate level of care, as they have during armed conflict and natural disasters.

But experts worry that most staff members are not trained to work in such settings, and that the lack of protective gear and ventilators vexing traditional hospitals could be worse in an exhibition hall turned into a crisis center. Another concern: that the makeshift spaces could further facilitate infection.

“As far as infection prevention, it’s always easier in small wards than big open spaces,” said Paul Hunter, professor of medicine at the University of East Anglia who helped health teams battle the Ebola epidemic by better managing their medical waste.

“When you are donning and doffing — putting on and taking off the protection clothes — there is more control, moving from one patient to another. It will certainly be outside the experience of most health-care professionals — and that, itself, will be stressful,” Hunter said.

But he added, “You know, you don’t go into medicine if you are fussy about the environments you work in.”

Some hospitals in northern Italy and in Spain — the centers of Europe’s most concentrated coronavirus outbreaks — have been inundated with so many patients that they’ve had to treat people in chaotic corridors or parking lots, or turn them away altogether.

To relieve the pressure on the system in Madrid, the Spanish military, with the regional government and health ministry, prepared a field hospital in 24 hours in the vast IFEMA convention center, with a current capacity of 1,300 beds for patients in stable condition and 96 intensive-care beds.

In December, the convention center hosted delegations from around the world for the U.N. Framework Convention on Climate Change (COP25) summit. It is now Spain’s largest hospital.

The patients “are comfortable and are receiving excellent care. It’s certainly better than waiting in a chair overnight in an emergency room waiting for attention,” said a doctor working at Madrid’s temporary hospital, who spoke on the condition of anonymity to be candid.

The doctor complained about disorganization and the lack of medical supplies at the city’s temporary hospital. “It’s frustrating and demoralizing,” he said. “But when you are dealing with the patients, it all fades away, because they are so grateful and appreciative. They know we are doing our very best.”

Frontline nurses and physicians throughout Europe and in the United States have complained about the scarcity of protective gowns, masks and gloves. Some nurses have donned garbage bags to protect themselves. Thousands have signed letters of protest.

And that’s before the full force of the virus has been felt in many countries.

Meng Aw-Yong, an emergency physician at Hillingdon Hospital, a 500-bed facility near Heathrow Airport, noted that models of the virus’s progression suggest Britain’s traditional NHS hospitals “will be overwhelmed, and it won’t be long.”

He said the Nightingale Hospital in London’s convention center is desperately needed: “It’s warm, it has electricity, it has power and it has lighting. And it has good infrastructure. A field hospital — as in a big tent — is not ideal. Having a facility like this is a really good solution.”

The hospital will more than double the 3,000 beds currently available in London. The NHS is building two more, equally large facilities, in Birmingham and Manchester.

Meng said the temporary hospitals “will be seeing a combination of sick patients and ICU. It won’t be entirely ICU. It will basically pull off what local hospitals cannot manage. Remember, we will still be dealing with people who have had heart attacks and other problems. The ExCel Centre will be used to help hospitals to keep functioning.”

France has been more skeptical about field hospitals, choosing instead to move patients from strained regions to areas where hospitals are not as full. Last week, a specially converted high-speed train transferred 20 patients from Strasbourg in eastern France to the Loire region in western France.

The limiting factor is not the size of hospitals. More beds are doable, said Martin Hirsch, the director general of Assistance Publique-Hôpitaux de Paris, the largest network of public hospitals in Europe.

“It's more the availability of personnel, of supplies and oxygen,” he said.

Allyson Pollock, a physician and professor of public health at Newcastle University, said Britain’s health system was dangerously underresourced going into this crisis.

“I would ask: Why we are in this position? Why do we have to expand our capacity in this way? The lesson for politicians is they cut health service at their peril,” Pollock said.

“We’ve had huge underinvestment, particularly since 2008 with the financial crisis,” she said. “In 2009, there was a warning — the pandemic flu report — that they needed to expand the ventilators. And no one did.”

Rolfe reported from Madrid. James McAuley in Paris and Loveday Morris and Luisa Beck in Berlin contributed to this report.