CDC's Updated Guidelines for Evaluating Public Health Surveillance Systems was used to assess the usefulness, simplicity, flexibility, data quality, acceptability, representativeness, timeliness, stability, sensitivity, and PVP of ESSENCE for the USAF.2 To determine sensitivity and PVP, medical record data from ILI case-patient visits at the USAF Academy acute care and cadet clinics during June 25–July 8, 2009, were collected. For the medical record review, ILI was defined as measured temperature ≥100.0°F (≥37.8°C) and cough or sore throat. Medical record data collection included cough, sore throat, measured temperature, and the results of respiratory disease laboratory tests for influenza A, influenza B, H1N1, adenovirus, and group A streptococcus bacteria. Patients from the same period were identified on the ESSENCE ILI website to collect the ICD-9-CM codes. Sensitivity and PVP were calculated using (1) medical record—confirmed ILI, as defined and (2) laboratory confirmation of a respiratory infection, as criterion standards. USAF Academy, USAF School of Aerospace Medicine (USAFSAM), DoD, and CDC staff members who used ESSENCE daily were interviewed to assess the remaining evaluation criteria.