The three primary ethical challenges in preparing for public health emergencies - addressing questions of rationing, restrictions and responsibilities - all entail confronting uncertainty. But the third, considering whether people and institutions will live up to their responsibilities in a crisis, is perhaps the hardest to predict and therefore plan for. The quintessential example of a responsibility during a public health emergency is that of health care professionals' obligation to continue caring for patients during epidemics. Historically, this 'duty to treat' has sometimes gone unrecognized or ignored, but it has also famously been adhered to, including during the recent SARS epidemic. And non-crisis examples of health professionals working in the face of personal risk are very common. The duty to treat should be circumscribed by several considerations, including the levels or risk and benefit at issue, the degree of public reliance on health professional action, and the nature of the individual health professional's acceptance of greater than usual risk. Examining the professional duty to treat and the legitimate questions it raises can provide insight into other actors' responsibilities. Public health ethics as well as professional ethics can help frame answers to some key questions: How strong are ethical responsibilities during crises? To whom do they apply? Should they be more explicit - and hence more circumscribed - or less explicit and hence largely aspirational? And how can public health policies encourage responsible actions?