The threat of bioterrorism, the emergence of the SARS epidemic, and a recent focus on professionalism among physicians, present a timely opportunity for a review of, and renewed commitment to, physician obligations to care for patients during epidemics. The professional obligation to care for contagious patients is part of a larger "duty to treat," which historically became accepted when 1) a risk of nosocomial infection was perceived, 2) an organized professional body existed to promote the duty, and 3) the public came to rely on the duty. Physicians' responses to epidemics from the Hippocratic era to the present suggests an evolving acceptance of the professional duty to treat contagious patients, reaching a long-held peak between 1847 and the 1950's. There has been some professional retrenchment against this duty to treat in the last 40 years but, we argue, conditions favoring acceptance of the duty are met today. A renewed embrace of physicians' duty to treat patients during epidemics, despite conditions of personal risk, might strengthen medicine's relationship with society, improve society's capacity to prepare for threats such as bioterrorism and new epidemics, and contribute to the development of a more robust and meaningful medical professionalism.