The incident in May-June 2007 involving a U.S. citizen traveling internationally while infected with drug-resistant tuberculosis involved the U.S. federal government's application of its quarantine and isolation powers. The incident and the isolation order raised numerous important issues for public health governance, law, and ethics. This article explores many of these issues by examining how the exercise of quarantine powers provides a powerful lens through which to understand how societies respond to and attempt to govern threats posed by dangerous, contagious pathogens. The article considers historical aspects of governmental power to quarantine and isolate individuals and groups; analyzes the current state of quarantine and isolation law in the United States in light of the recent incident with drug-resistant tuberculosis; and explores global aspects of public health governance and law highlighted by this incident.