Nearly 10 years ago, we published in the Harvard National Security Journal a long article about preventive detention—that is, detention justified in law by the need to prevent future harm, rather than as punishment for a past crime. After examining more than a dozen varieties of preventive detention allowed by American law, we concluded that, despite the “civic myth” that preventive detention is disfavored and rare, it is neither prohibited nor “especially frowned upon in tradition or practice.” Surprisingly, American law allows quite a lot of it. One of the areas we examined was the power of quarantine and isolation to protect the public health. Unfortunately, that power is newly relevant—more relevant than ever before in the living memory of most Americans. Several basic themes emerge from a survey of the history and law of quarantine in the United States. First, involuntary measures to control disease have a long history in North America.…