The outbreak of SARS (2002-2003) makes an interesting case study which illustrates the nexus of infectious disease and global security. Four aspects of the epidemic are particularly significant: the character of the newly emergent pathogen, the unprecedentedly rapid tempo of its worldwide spread, the transformed global public health context in which it occurred, and the potential of microbes to affect state policies. SARS was but the first of the serious new diseases to have emerged in the twenty-first century, followed, as it has been, by H1N1 (2009), Ebola (2009), Middle East respiratory syndrome (2015), and Zika (2015), among others. It reaffirmed what was already known: that the changing patterns of human ecology and behavior (such as a taste for exotic food and urbanization) have been important triggers for the development of new zoonotic diseases. As pathogenic microbes naturally continue to evolve and at times cross the Darwinian divide between species, SARS conveys a salutary lesson.