Chris Christie and Andrew Cuomo are playing doctor. The governors of New Jersey and New York on Friday announced a mandatory quarantine for health care workers who have recently treated Ebola patients in West Africa. Upon returning to the U.S. through JFK or Newark Airports, those workers must now spend 21 days in isolation. State officials are still working out key details, but it sounds like most workers would spend that period inside their homes, although some could end up at medical facilities.

The announcement came one day after Craig Spencer, a physician with Doctors Without Borders, became the fourth person to be diagnosed with Ebola in the U.S. After returning from Guinea on October 17 and going through the enhanced screening at JFK Airport, Spencer had mostly stayed in his apartment. But on Wednesday, the day before he developed a 100.3 fever, Spencer had taken the subway and bowled at an alley in Brooklyn. By imposing a mandatory quarantine, Christie and Cuomo said on Friday, they could prevent such situations in the future.

Are they right? And is a quarantine worthwhile? It's a complicated question. The Centers for Disease Control and most public health experts that I've consulted do not recommend taking such steps because, they say, they are not medically necessary. Ebola patients don’t become contagious until they have real symptoms, such as fever or vomiting. It’s enough, these experts say, if people at risk of Ebola “self-monitor”—i.e., watch out for symptoms and take their temperatures twice a day, reporting in any problems to public health authorities. That’s precisely what Spencer did and, according to every expert I’ve consulted or seen quoted, it’s highly unlikely he gave the disease to anybody. Spencer is now in a special isolation facility at Bellevue Hospital, getting treatment.

Of course, the argument for quarantine may be less about medicine than it is about mass psychology. Ebola is a scary disease and quarantine's main, perfectly worthy goal may simply be to calm the public. But in public health, as in medicine, the first principle is Do No Harm. Medical experts and health officials worry, for example, that quarantine might discourage aid workers from traveling to West Africa, at a time when the region desperately needs more personnel to fight the epidemic. 

David W. Chen and Liz Robbins the New York Times interviewed health care workers who have spent time working in West Africa:

While a few of those interviewed said an overabundance of caution was welcome, the vast majority said that restrictions like those adopted by New York and New Jersey could cripple volunteers’ efforts at the front lines of the epidemic. ... 

Dr. Rick Sacra, who contracted Ebola in Liberia and was flown back to the United States to be treated in September, said he believed that the new rules in New Jersey and New York would reduce the number of people willing to volunteer their time to treat Ebola patients. 

He said many doctors and nurses who volunteered would spend about three weeks in Africa and then return to their regular jobs. The requirement that they be quarantined at home upon their return “will effectively double the burden on those people, on the loss of productive time,” Dr. Sacra said. 

It's also an open question whether the quarantine reduces anxiety or intensifies it. That's particularly true in this case, because Cuomo’s statements on Friday, at least as relayed by the press, left the impression that a non-symptomatic Ebola patient could spread the disease on the subway—the very notion that public health officials had spent the previous 24 hours explaining wasn’t true.

That’s one reason that officials from the Obama Administration, the CDC and the New York City Department of Public Health seemed not at all happy about Friday’s announcement. The other is that, based on what I’m reading in outlets like the Times and hearing from insiders, they weren’t so much consulted about the decision as informed of it at the last minute, as a fait accompli. Leaders in all three of those places pride themselves on putting science before politics. You don’t have to be a cynic to think that Cuomo, who is up for reelection, and Christie don’t feel the same compulsion.

None of this means that Christie and Cuomo are clearly wrong on the merits. Truth is, this is one of those questions about which reasonable people can disagree or have mixed feelings. (Here, for example, is a prominent infectious disease specialist arguing for an even wider quarantine.) But it's probably safe to assume that the two governors weren't simply thinking about public health when they made their decisions. They were thinking about public perceptions—for the best reasons, the worst reasons, or maybe some of both.

Update: I added a line clarifying that opinion on the quarantine was based heavily on my own interviews.