China was the first country to forcibly restrict the personal movements, public events, and business activities of its people—eleven million in Wuhan, the megacity and global epicenter of the COVID-19 coronavirus. On January 23rd, soldiers wearing black masks deployed along barricades at the train station. The government requisitioned stadiums, exhibition halls, and other large venues as quarantine centers; the rows of beds set up inside looked more like military barracks than hospital wards. To contain the largest outbreak in Asia, Chinese health and security officials went door-to-door—checking for people with symptoms. The lockdown was later widened to more than a dozen other cities in Hubei Province, constraining the daily lives of sixty million people. It’s the largest quarantine in human history.

On Sunday, Italy locked down its northern regions—including Venice and Milan, the country’s economic heartland, and home to sixteen million people—for at least a month to contain the largest outbreak in Europe. Nationwide, all public gatherings, including weddings and funerals, were banned. In an unprecedented emergency decree, Prime Minister Giuseppe Conte told a news conference—convened at 2 A.M.—that Italy faced “a national emergency.” Local leaders—some given little advance warning and others who learned on television—complained that the plan was incoherent and enforcement unfeasible. But the government said it had little choice. In just twenty-four hours, Italy recorded fourteen hundred new infections. One of them was Nicola Zingaretti, the leader of the governing coalition’s Democratic Party, who is based in Rome, far from the COVID-19 front line in Italy. “Well, it’s arrived,” he said, in a Facebook video. Schools nationwide had already been closed. The Vatican, the headquarters of the world’s 1.2 billion Catholics, was, too. On Monday, Conte announced a nationwide lockdown to go into effect on Tuesday morning; it will apply to roughly sixty million people.

In just ten weeks, the number of reported cases of the COVID-19 coronavirus has soared to more than a hundred and ten thousand in a hundred countries. In the United States, thirty-four states now have cases. The first case in Washington, D.C., reported over the weekend, is the rector of Georgetown’s historic Christ Church, who led four services for five hundred and fifty people a week ago; he handed out communion at one of them and spent part of the service reminding the congregation about hand sanitizers. After he was hospitalized on Thursday, the church e-mailed alerts to parishioners and cancelled services yesterday.

The great unknown is whether the kinds of restrictions imposed in China and Italy are harbingers of a global phenomenon, including in the United States. Governments on six continents are scrambling to figure out how to stem community contagion, which is now widespread but hard to trace backward to contain the infection rates. Will parts of the world be quarantined in enclaves until a vaccine is found, tested, approved and available—at least a year away?

“We have to be realistic. I don’t think it would be as draconian as nobody in or nobody out. But, if we continue to get cases like this, particularly at the community level, there will be what we call ‘mitigation,’ where we have to essentially do social distancing,” Dr. Anthony Fauci, the head of the National Institute of Allergy and Infectious Diseases, said on Fox News Sunday. “You don’t want to alarm people, but, given the spread we’ve seen, you know, anything is possible. And that’s the reason why we’ve got to be prepared to take whatever action is appropriate to contain and mitigate the outbreak.”

At the same time, no country can simply quarantine its way out of the COVID-19 crisis, Wendy E. Parmet, the director of the Center for Health Policy and Law at Northeastern University, told me. “There are reasons to be skeptical of the efficacy of quarantine for respiratory diseases like coronavirus.” Quarantines can be a useful tool when done well. They can lower infection rates “a bit” and buy time, she said. But they have been done historically in discriminatory and haphazard ways that provide “a seductive illusion of containment.” The best example is the disproportionate U.S. focus on the long-stranded Grand Princess cruise ship, which has more than twenty passengers who have tested positive for COVID-19. It is finally due to dock in Oakland on Monday; passengers will be quarantined for two weeks in California, Texas, and Georgia. Meanwhile, Parmet said, the disease has spread widely and rapidly across most of the United States.

There are three distinct responses to health emergencies that have been misused interchangeably amid the coronavirus crisis, Lauren Sauer, of the Johns Hopkins Office of Critical Event Preparedness and Response, told me. Social distancing, a term that has just entered the modern lexicon, is voluntarily keeping a few feet away from other people or opting not to go to mass gatherings based on the recommendation of public-health officials. It is the least restrictive—and the strategy that U.S. officials have largely recommended so far, especially for vulnerable sectors of society.

A quarantine, a practice first recorded in the Book of Leviticus, in the Old Testament, is invoked because of probable exposure to an illness. People are removed from the general public for a period of time—voluntarily or by legal order—to see if the disease develops. The word quarantine dates to the Black Death’s spread along the Mediterranean in the fourteenth century, when governments imposed isolation for forty days—or quaranta in Italian. The Black Death killed some twenty-five million people, about a third of the European population at the time. Italy and China have mandated quarantines in response to COVID-19. In the United States, Senator Ted Cruz, of Texas, and Representative Paul Gosar, of Arizona, announced on Sunday that they are self-quarantining for fourteen days, because they interacted and shook hands with an unidentified person at the Conservative Political Action Conference last month who subsequently tested positive for the new virus.

The most restrictive measure is isolation after a person has symptoms or is confirmed to have a disease by a lab test. “In quarantine, you’re waiting to see if you develop the sickness,” Sauer said. “If you’re in isolation, you are sick and waiting until the illness resolves.”

So far, countries are reacting in vastly different ways to the COVID-19 epidemic. One increasingly common practice has been quarantining passengers returning from so-called hot zones, or countries with numerous coronavirus cases, including China, Italy, Iran, South Korea, and Japan. But, with more than a hundred countries now hit, the requirements needed to effectively quarantine the volume of passengers from so many places could overwhelm systems. Prime Minister Benjamin Netanyahu said on Sunday that Israel, which has more than three dozen cases, is considering a move that would force both Israelis and foreigners flying in from anywhere in the world to go into quarantine. The consequences and enforcement mechanisms have been inconsistent, however. In one of the toughest measures, the Czech government decreed that any of its sixteen thousand citizens in Italy have to self-quarantine for fourteen days when they return—or face a hundred-and-thirty-thousand-dollar fine.