The word "quarantine" would seem to have a straightforward definition: It's when a sick person is shut in for a period of time—perhaps with yellow tape or a sticker on the door—until he or she is cured or can no longer spread the disease.

But with the Ebola epidemic, the meaning of quarantine is not nearly so clear.

That's partly because Ebola patients aren't contagious until they've started exhibiting symptoms such as fever, vomiting, and diarrhea. Someone may be incubating the disease for a week or more without knowing it, potentially putting others at risk. And yet locking up everyone returning to the U.S. from the three Ebola-stricken countries in West Africa seems pointless if they can't pass on the disease anyway.

On