BEIJING - Chinese health authorities are extending the holidays and deploying more than a thousand doctors and military personnel to the epicenter of the coronavirus outbreak as the number of infections skyrocket and desperation grips the quarantined province of Hubei, where more than 50 million people are stranded with a severe shortage of medical supplies.

The United States, meanwhile, announced a third case of the coronavirus - a Chinese traveler from Wuhan who took ill in Orange County, California. Authorities say he is under care in isolation and the "risk of local transmission is low."

China's national health commission said early Sunday that the number of confirmed infections had soared 50 percent over the prior 24 hours to 1,975 people across 30 provinces. Fifty-six deaths have been reported, including in major metropolitan areas such as Shanghai. Several doctors in Beijing, the capital, also reported being infected.

"Transmissibility is increasing," Chinese Health Minister Ma Xiaowei told reporters Sunday. "The outbreak has come to a severe and complicated situation."

He added that there could "still be new developments" as the virus mutates. "We still don't know the risks of transformation," he said.

Scientists have already noticed that the virus is adapting to humans much faster than its predecessor, the SARS (severe acute respiratory syndrome) outbreak, which killed more than 750 people in 2002-2003.

It took SARS three months to mutate into a form that spread easily between humans, but the related Wuhan coronavirus took only one month, George Fu, a top Chinese epidemiologist told reporters.

"Why is it transmitting so fast?" he said. "The two species are like the cartoon Tom and Jerry: viruses are continually adapting to humans, but human also adapt, and the virus' ability to make people ill also goes down."

A government group led by Premier Li Keqiang handling the epidemic response on Sunday proposed a "reasonable extension" of the New Year holiday in order to weather a "vital phase in epidemic prevention and control." Cities are already rolling out such measures: Beijing's education officials said schools will resume classes in mid-February, while the manufacturing hub of Suzhou prohibited large businesses from reopening earlier than Feb. 8.

At the heart of the outbreak, in central China's Hubei Province, a travel ban extended to a total of 16 cities and covered approximately 51 million people. Video distributed by state media showed local officials in adjacent regions taking extreme measures, including using excavators to destroy and block roads, to discourage residents from traveling to infected areas of Hubei to visit stranded relatives - as per Chinese tradition - inside the quarantine zone.

Authorities have also announced the banning of the sale of wild animals after evidence emerged that the disease was transmitted to humans through a market in the city of Wuhan that traded in game meat.

The spread of the virus - and travel bans extending to several major hubs around China - threatened to paralyze the country for an indefinite period, with uncertain implications around the world. Officials in Beijing said Sunday they "have not and will not close the city because of the epidemic" in response to online rumors suggesting an imminent lockdown of the capital, which has a population of 22 million, with a significant fraction traveling this week to visit family.

On Saturday - China's New Year's Day - numerous Chinese government agencies said they had summoned workers back to their posts as President Xi Jinping warned of a "grave" situation as the virus "accelerated its spread."

Two teams of British epidemiologists released studies over the weekend estimating that each infected person was spreading the disease to two or three other people. A team from Lancaster University projected that infections in Wuhan could explode to 190,000 cases by as early as next week.

The Chinese central government said it is mustering manufacturers to send 100,000 hazardous materials suits and millions of face masks to Wuhan, where hospitals reported overfilled beds and doctors collapsing from exhaustion. Videos on social media from Wuhan hospitals showed patient queues stretching around the block and nurses surmising that the true number of cases - based on what they were witnessing - far exceeded what was being officially reported.

The vice minister of industry, Wang Jiangping, said Sunday the country was facing a significant shortage of medical supplies including protective suits for medical workers. Hubei Province alone required 100,000 suits a day but Chinese manufacturers could only produce 30,000 a day, he said. "There's a prominent gap in supply and demand," he said, adding that China was hoping to purchase supplies on the international market.

Masks in particular have been in short supply, with shortages now being reported abroad as well. In Japan, Chinese tourists have been emptying the shelves of face masks, according to local news reports, while supplies are also running out globally.

Michael Einhorn, president of Dealmed, an independent medical supply distributor in New York, New Jersey, Connecticut and Pennsylvania, said prices of masks jumped as soon as news of the virus spread, while sales volumes tripled through retailers such as Amazon.

Dealmed typically carries at least 90-days' supply, and but said the company currently only has about two weeks' worth left.

If demand continues, the availability of masks will be "very limited" within the next 10 days, and of the virus continues to spread there will be "extreme shortages" as early as three weeks' time, Einhorn said in a statement.

Authorities in Wuhan and another hard-hit Hubei city, Huanggang, have announced the construction of three pop-up hospitals with thousands of beds to be built within the next few days. Ma, the health minister, said 5,000 new beds should be available by midweek while hundreds of medical professionals are preparing to deploy to the region.

Days after ordering the departure of all non-emergency U.S. personnel, the U.S. Embassy said Sunday it would charter a single flight on Jan. 26 out of Wuhan for remaining consulate staff and American citizens.

Japan has also said it is readying flights to bring home more than 700 Japanese stranded in Wuhan.

Meanwhile, Chinese citizens stranded inside the vast quarantine zone, locked down by paramilitary police checkpoints for the fourth day, took to social media to describe a sense of surreal desperation during a week when families should otherwise be celebrating the new year with dumplings, fireworks and presents.

One Wuhan resident described sharing her dwindling groceries that she had purchased to last for three days with an elderly couple whose food supplies were down to nothing. She said she worried about her food lasting one more day and the population of stray animals abandoned throughout the city.

"I don't know how to solve this food problem," wrote the user "Guapidawushi. "Right now I really, really don't know what to do. I'm completely helpless."

Some users shared videos of once-buzzing streets in Wuhan's historic, European-style riverside district lying empty. Others posted more lighthearted pictures of women playing Mah-jongg with masks and transparent grocery bags over their heads.

The situation appeared to be more dire in the vast Hubei countryside outside of Wuhan, where rural authorities were struggling to cope.

The Chinese magazine Caijing reported that some smaller village clinics were only rationed six masks, and large hospitals were within one or two days of running out of supplies. In Jingzhou city, a short distance up the Yangtze River from Wuhan, doctors told reporters they were wearing rain ponchos because they lacked protective suits.

Deng Anqing, a Beijing-based writer who was visiting family in rural Hubei for the New Year holiday, said the hidden crisis was in the countryside.

"The media is focused on Wuhan but we know absolutely nothing about the current situation in the countryside," Deng wrote in a post. "Large numbers of workers are returning here from Wuhan, but the capabilities of village hospitals are awful. Villages don't have masks, and it's hard to convince the elderly to wear them."

Infections have been confirmed in France, Australia and the United States, but countries in Asia have been especially concerned as millions of Chinese people fan out across the region for the Lunar New Year.

An online petition asking South Korean President Moon Jae-in to ban Chinese nationals from entering the country has drawn more than 280,000 signatures over four days. South Korea confirmed a third case of coronavirus infection on Sunday, a 54-year old man who returned from Wuhan.

North Korea's official party daily Rodong Sinmun posted prevention advice on Sunday and called for stronger border controls in a piece headlined "We should thoroughly contain the new coronavirus." Earlier this week, North Korea banned all foreign tourists, most of whom come from its biggest neighbor and ally China. Flights between Beijing and Pyongyang have also been canceled, the Russian Embassy in North Korea said in a statement on Friday.

Japan confirmed it's fourth case of the virus, a middle aged man from Wuhan who arrived on Wednesday for vacation.

In Hong Kong, where a sixth case of the virus was confirmed Sunday, pressure is mounting on the government to tighten border controls with China over fear of contagion. One hospital workers' union threatened a five-day strike if more measures weren't taken.

Protesters in the evening attacked a building that has been set aside for quarantine and set its lobby alight with molotov cocktails, police reported.

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Denyer reported from Tokyo. The Washington Post’s Min Joo Kim in Seoul, Shibani Mahtani in Hong Kong, Lyric Li in Beijing and Paul Schemm in Dubai contributed to this report.