More than 100 people have died from a coronavirus outbreak that started in Wuhan, China, and more than 4,600 people have been infected across 17 countries.

The virus, which is marked by fevers and pneumonialike symptoms, likely originated in a wet market in Wuhan, an 11 million-person city in China's Hubei province.

On January 23, authorities put Wuhan under quarantine — halting all public transportation, including city buses, trains, and ferries. The order prevents any buses or trains from coming into or leaving the city and grounds all planes at the Wuhan airport. Wuhan authorities started to limit car travel the next day as well, The Guardian reported.

The city of Huanggang (which is home to around 7.5 million people) also went into lockdown last week, as authorities closed subway and train stations. By the following day, 10 additional cities — Chibi, Enshi, Ezhou, Huangshi, Suizhou, Qianjjiang, Xianning, Xiantao, Yichang, and Zhijiang — had followed suit with their own travel restrictions. As of Monday, the cities of Xiangyang, Jingmen, Xiaogan, and Dangyang were also quarantined.

Tedros Adhanom Ghebreyesus, the director-general of the World Health Organization, said these efforts to quarantine cities will help Chinese authorities control the virus' spread and "minimize the chances of this outbreak spreading internationally."

"What they're doing is a very, very strong measure, and with full commitment," Ghebreyesus said in press conference last week.

Quarantining 50 million people

The Beijing railway station on January 21. Kevin Frayer/Getty

All told, these restrictions affect an estimated 50 million people, according to The Washington Post.

Wuhan is China's fifth-largest city and more than 3,200 square miles in size. By comparison, New York City has about 8 million people — nearly 3 million fewer than Wuhan.

The city is split into three districts: Hankou, Wuchang, and Hanyang, with rivers cutting between them. Many people work in one district and live in another, so the ban on interdistrict travel cripples the city's normal functioning.

According to local authorities, citizens are no longer permitted to leave Wuhan without a "special reason." At noon local time Friday,

"To my knowledge, trying to contain a city of 11 million people is new to science," Gauden Galea, the WHO's China representative, told the Associated Press on Thursday. "It has not been tried before as a public-health measure, so we cannot at this stage say it will or will not work."

In Huanggang, meanwhile, officials ordered cinemas and internet cafes to close and told citizens not leave the city unless there are special circumstances, Reuters reported. The city's long-distance buses and trains were halted. The nearby city of Ezhou, too, closed its train stations, while bus travel in Chibi and Zhijiang, two smaller cities, has been stopped.

These quarantines come amid the Lunar New Year celebrations, which began on Saturday. Usually, the annual holiday fills the country's trains, planes, roads, and ferries, as 3 billion people journey home to be with family. However, the Chinese government has asked people to cancel plans to visit Wuhan, and Beijing's government canceled its Spring Festival events last week.

International efforts to stop the coronavirus outbreak

Public-health officials run thermal scans in Bangkok, Thailand. Lauren DeCicca/Getty Images

Five US airports — New York's John F. Kennedy International Airport, San Francisco International Airport, Los Angeles International Airport, Hartsfield-Jackson Atlanta International Airport, and Chicago O'Hare International Airport — are screening passengers for the virus.

Airports in Hong Kong, Singapore, Thailand, and South Korea are also checking passengers for fever.

Travelers leaving Wuhan before the quarantine went into effect all had their temperatures checked.

But Eric Toner, a senior scientist at Johns Hopkins University, told Business Insider that given the number of coronavirus cases already, "any sort of travel restrictions are likely to be ineffective."

On Monday, the mayor of Wuhan, said about 5 million people had left Wuhan before the city was placed on lockdown.

A mother holds her child at a railway station in Beijing, China. Kevin Frayer/Getty Images

Bart Haagmans, a virologist at the Erasmus Medical Center in Rotterdam, Netherlands, told Business Insider that one of the challenges in containing the outbreak is that a substantial fraction of infected people show only mild symptoms.

These people "may go unnoticed in tracing the virus and fuel the outbreak," he said, adding, "It seems that this actually may be the case now."

Aria Bendix and Ryan Pickrell contributed reporting to this story.