What Is a Pandemic?

The sudden emergence and rapid global spread of a novel H INI influenza virus in early 2009 [ 1 ] has caused confusion about the meaning of the word and how to recognize pandemics when they occur. Any assumption that the term pan demic had an agreed-upon meaning was quickly undermined by debates and dis cussions about the term in the popular media and in scientific publications [2-5]. Uses of the term by officiai 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 infec tion should also be considered [2-5]. Commentators questioned whether we could effectively deal with a pandemic when we could not agree on what a pan demic is or whether we were experiencing one. Amid this discussion, a New York Times commentary, published 8 June 2009, struck at the heart of the problem with its challenging headline, Is This a Pandemic? Define 'Pandemic' [5]. Three days later, the World Health Organization (WHO) announced that the pandemic