For the purpose of CDC influenza surveillance, ILI is defined as a fever $37.8˚C and a cough and/or a sore throat without known etiology (http://www.cdc.gov/flu/weekly/fluactivity.htm, accessed 09/04/09). ILI is not specific to influenza, however. Prospective studies with laboratory sampling of persons with ILI have demonstrated a wide variability in the specificity of ILI for influenza disease, with the proportion of subjects testing positive for influenza ranging from 20% to 70% of those tested during the influenza season [3, 4] . ILI may also not be sensitive for influenza, particularly in certain age or risk groups where influenza may have atypical presentations [5, 6, 7, 8, 9] . Furthermore, even during peak periods of influenza circulation, a substantial number of cases of febrile respiratory illness may have non-influenza etiologies. In the United States, during the spring wave of the 2009 H1N1 outbreak from March through August 2009, the proportion of positive influenza laboratory tests did not exceed 45% (http://www.cdc.gov/flu/ weekly, accessed 09/04/09).