The deadly Ebola virus — with the horrific symptoms it causes (profuse vomiting and diarrhea, bleeding gums and other unexplained bleeding), its lack of medications or vaccines, its rapid spread and its gruesome fatality rate (an estimated 70% in West Africa) — conjures images of ancient plagues that depopulated major cities, paralyzed trade and travel, trampled individual rights and sowed famine and social upheaval. The current Ebola epidemic started with a two-year old child who died December 6, 2013, in Guinea and has now killed an estimated 6,300 people in six West African nations. If not contained there, it could become a global pandemic.

As usually happens in the face of such a contagion, government and public health officials are using one of the few tools at their disposal—a quarantine—to try to halt the spread of the disease. Quarantines isolate and restrict the movement of people who have communicable diseases or have been exposed to them. During Europe’s Black Death (bubonic plague) in the 14th century, for example, cities and towns cordoned off roads and rivers to keep people out, put up chains around neighborhoods to keep the contagious in and detained and inspected travelers — often on pain of death or imprisonment. (The word quarantine itself derives from quaranta, Italian for 40, the number of days that ships were delayed before being allowed into the Venice harbor.)

But quarantines are blunt instruments and have always been controversial, says Howard Markel, a historian of medicine at the University of Michigan. They raise ethical dilemmas about the responsibilities of people who may be carrying a disease; conflicts between the rights of the individual and the greater public good; discrimination against “undesirables” for allegedly spreading disease; distrust of public health officials; scorn for experts; public demand for action; internecine conflicts between local and national (and now international) authorities. Quarantines also spread their own contagion—of fear.

Even so, quarantines are often the best public health tool available, and can indeed help contain infection and delay its spread, especially when there’s not much in the way of effective treatments, according to Eugenia Tognotti, a historian of medicine at University of Sassari in Italy. But effectiveness depends on the attributes of the epidemic you’re trying to contain, and quarantines have changed as we’ve learned more about the particular pathogens that cause a disease, how they spread and how long an infected person may be contagious. For the plague, a 40-day quarantine was long enough for the fleas carrying the plague bacterium to die, so they couldn’t infect more people. But for cholera, caused by contaminated food and water, improving sanitation worked better than quarantines.

In Africa, both now and during the first Ebola outbreak in 1976, quarantines have been a major component of the public health response to the disease—and they’ve often succeeded in helping contain it. During that first Ebola epidemic, in Zaire (now Democratic Republic of Congo), quarantining some 250,000 people in huts outside of villages helped limit the spread of disease, according to two physicians involved in the effort. Officials used mandatory surveillance and home visits, rigorous identification of patients’ contacts and strict isolation of anyone who had symptoms of the disease. And during the current outbeak, Mali moved quickly to track down those who had come into contact with an infected two-year-old girl all along a 1,200 kilometer (745 mile) journey from Guinea to Mali. Officials monitored those most at risk for 21 days and appeared to have stopped Ebola in its tracks, although another, unrelated case then appeared, and additional patients and health workers have been quarantined. The affected clinic has reportedly been surrounded by police officers and nearby United Nations peacekeeping armored vehicles.

All over West Africa, such labor- and resource-intensive responses have provided the best hope of (or at least a stopgap measure for) limiting the destruction and spread of Ebola. Governments there don’t have the financial or medical resources—miracle drugs or state-of-the-art equipment and facilities—to mount the kinds of countermeasures that the U.S. and other developed countries have taken.

Yet it’s in Africa that Ebola has taken hold, and while quarantines there have been reasonably effective, their use globally may have perverse consequences. According to international aid workers, if the West wants to avoid the nightmare scenario of having the virus escape Africa and eventually inundate and cripple the developed world, the U.S. and other nations need to send 5,000 additional nurses and doctors to West Africa. Doing that, rather than focusing so much on the smattering of cases those countries have seen, would likely have a far greater impact. But the threat of being quarantined and stigmatized when they return home, as much as their risk of death on the front lines of Ebola treatment, could deter some health workers from going and contribute to the epidemic’s spread.  And in fact there may be no need, at least in the U.S., for mandatory quarantines.

A decisive factor in the rationale for a quarantine is how and whether a disease is transmitted from people who are contagious but not yet sick, explains Tognotti. “Quarantines are most beneficial when there is significant asymptomatic transmission and if the asymptomatic period is neither very long nor very short,” Tognotti says. But Ebola doesn’t have an asymptomic transmission period at all — people with the virus are contagious only if they’re experiencing active symptoms such as fever, diarrhea, vomiting, bleeding and malaise. You risk developing Ebola only if you have direct contact with an infected person’s blood, vomit, feces, urine, saliva, sweat or other body fluids. After that, it normally takes about 21 days to develop symptoms, and only then could you spread the disease.

That means that the best approach is a 21-day period of monitoring—rather than quarantining—those who’ve had contact with Ebola patients, looking for the earliest symptoms of fever, diarrhea, fatigue and other signs of the disease. When and if those appear, the affected person can be whisked into a modern, fully equipped Ebola treatment unit. U.S. guidelines also call for commonsense, individualized limits on travel and public outings during that monitoring period.

But not everyone will follow commonsense guidelines. A doctor who returned from caring for Ebola patients on October 17 and who felt “fatigued” nevertheless rode the New York City subway the day before a fever drove him to the hospital for Ebola treatment on October 23. That incident led some states to defy federal guidelines and order mandatory quarantines for returning health care workers. The first nurse ensnared by that quarantine then refused to be isolated in order to defend her individual civil rights—fueling additional panic.

“It’s ironic that in the U.S., some of the greatest episodes of anxiety concerned health care professionals who had treated Ebola patients, been potentially exposed to Ebola, or even contracted Ebola,” Markel says. “The people treating Ebola patients are, of course, heroic individuals. But it is ironic that although health care workers take great pains to calm people down and allay fears about disease, great fears were unleashed surrounding the cases of two nurses from Dallas, the physician in New York, and the nurse quarantined in New Jersey. This, I think, was most unfortunate because the fights over U.S. quarantines tended to distract Americans  from focusing on the big issue: the epidemic in West Africa.”