“This is just mind-boggling: This is the mother of all quarantines,” said University of Michigan medical historian Howard Markel. “I could never have imagined it.”

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Cordoning off a region like China’s Hubei province pens the sick together with the uninfected. It increases the burden on authorities, who must ensure the flow of food, water and other supplies to the quarantined area. It is nearly impossible to enforce. And it sows distrust in government at a time when public support is essential, prompting people to evade the restrictions or refuse to report their symptoms, experts said.

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“The first and golden rule of public health is you have to gain the trust of the population, and this is likely to drive the epidemic underground,” said Lawrence O. Gostin, a professor of global health law at Georgetown University. “The truth is those kinds of lockdowns are very rare and never effective.”

From a practical standpoint, “where does one draw the line?” asked Leana Wen, the former health commissioner for the city of Baltimore. “Many people work in the city and live in neighboring counties, and vice versa. Would people be separated from their families? How would every road be blocked? How would supplies reach residents?”

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“We worked on numerous contingency plans to respond to outbreaks and other public-health crises,” she added. “To my knowledge, our health department had not considered a citywide quarantine.”

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With 41 people confirmed dead and more than 1,000 infected, China has cut all transportation links into and out of Wuhan — a city of 11 million where the new virus originated — and 13 other cites across the center of the country, as celebrations of the Lunar New Year began Friday. Normally, 400 million people could be expected to travel across Asia during the Spring Festival, but the government has adopted a drastic course of action in the face of the health crisis.

“They’re doing it because people who are in political leadership always think that if you do something dramatic and visible that you’ll gain popular support,” Gostin said. “They couldn’t have any sound public health advice.”

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The Chinese may gain some advantage from the quarantine if it helps reduce travel and increases “social distancing,” a reliable tactic in the spread of infectious disease that means keeping people apart as much as possible, said Tom Frieden, former director of the U.S. Centers for Disease Control and Prevention.

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“Like many things, it depends on how you do it,” Frieden said. “Cordoning off a city would have a positive impact if that reduces travel and the amount of mixing you have.”

Recent quarantine attempts have met with little success. During the Ebola epidemic of 2014, an attempt to cordon off the West Point slum in Monrovia, the capital of Liberia, sparked violent clashes and evasion of the lockdown, including by residents who bribed their way in and out. During the same crisis, Sierra Leone locked down most of its population for three days, which may have led to underreporting of symptoms.

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But a five-day shutdown of businesses across Mexico may have helped slow the spread of the H1N1 swine flu there in 2009.

More than a century ago, authorities tried but quickly abandoned partial quarantines of Philadelphia and Chicago during the 1918 flu pandemic that killed as many as 40 million people worldwide and more than a half-million in the United States.

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During the second wave of that epidemic, however, a few tiny communities, such as a naval base in San Francisco Bay and the mining town of Gunnison, Colo., were able to block the flu by voluntarily cutting off all contact with the outside world, according to a 2006 study led by Markel.

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Quarantining millions of people may be easier in an authoritarian country like China. But even in the United States, authorities have broad power to order quarantines as a last resort. In a 2007 report to the CDC on responding to a flu pandemic, an expert panel makes no recommendation to use mandatory lockdowns, even in the most severe, or “Category 5” outbreak — one in which an astronomical 2 percent of those diagnosed with an infection die.

Rather, the report says authorities should rely on voluntary isolation of the sick at home; voluntary quarantine at home of uninfected people with sick family members; limiting activities in schools; limiting or banning mass gatherings; social distancing; and encouraging people to work from home.

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These would be added to typical public-health measures such as surveillance of the spread of the disease and tracing the contacts of sick people.

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In 2006, the Homeland Security Council and the CDC released a 233-page strategy that warned a modern pandemic could kill as many as 200,000 to 2 million Americans.

The unlikely use of “geographic quarantine” would result in “the detention, within an epidemic zone, of persons who may or may not have been exposed to the pathogen in question,” according to the report.

But the National Strategy for Pandemic Influenza noted that a virus is likely to quickly surface in more than one location. When that occurs, the success of a geographic quarantine would be “profoundly limited,” effectively making it a remote option for curbing a pandemic.

“The implementation of conventional geographic quarantine imposes significant opportunity costs and may result in the diversion of significant resources and assets that might be used to better effect supporting less draconian disease-containment measures,” according to the report.

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Still, the document called on state and local governments to devise their own pandemic strategies that at least include the possibility of a widespread quarantine — one that would probably be ordered by state or local governments and enforced by National Guard troops if necessary.

“If directed by the President pursuant to the Insurrection Act, the military may suppress domestic unrest associated with resistance to a state quarantine,” the document stated, referring to the 1807 law that outlines presidential power to combat domestic unrest.

States have developed their own plans for responding to pandemics. A 2014 report by the Illinois Department of Public Health declares that the state has “absolute authority in matters of quarantine and isolation, and may declare and enforce quarantine when none exists.”

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But the document says the practice would most likely be used only in highly localized settings, such as restricting the movements of a group of nursing home residents or airline passengers who may have been exposed to a disease but are not yet symptomatic.