The sudden emergence and rapid global spread of a novel H1N1 influenza virus in early 2009 [1] has caused confusion about the meaning of the word “pandemic” and how to recognize pandemics when they occur. Any assumption that the term pandemic had an agreed-upon meaning was quickly undermined by debates and discussions about the term in the popular media and in scientific publications [2–5]. Uses of the term by official health agencies, scientists, and the media often seemed to be at odds. For example, some argued that a level of explosive transmissibility was sufficient to declare a pandemic, whereas others maintained that severity of infection should also be considered [2–5]