Responses to public health emergencies can entail difficult decisions about restricting individual liberties to prevent the spread of disease. The quintessential example is quarantine. While isolating sick patients tends not to provoke much concern, quarantine of healthy people who only might be infected often is controversial. In fact, as the experience with severe acute respiratory syndrome (SARS) shows, the vast majority of those placed under quarantine typically don't become ill. Efforts to enforce involuntary quarantine through military or police powers also (...) can backfire, stoking both panic and disease spread. Yet quarantine is part of a limited arsenal of options when effective treatment or prophylaxis is not available, and some evidence suggests it can be effective, especially when it is voluntary, home-based and accompanied by extensive outreach, communication and education efforts. Even assuming that quarantine is medically effective, however, it still must be ethically justified because it creates harms for many of those affected. Moreover, ethical principles of reciprocity, transparency, non-discrimination and accountability should guide any implementation of quarantine. (shrink)