Automated surveillance systems based on chief complaints and diagnoses have a number of uses: providing assistance in data collection; monitoring seasonal infl uenza (15) ; monitoring total ED visits during a crisis; and monitoring simple surrogates of infectious diseases, injuries, and chronic diseases during large outbreaks or disasters (16) . The utility of these systems has not been demonstrated for Additional counts Sensitivity Figure 2 . Sensitivity of detecting various numbers of additional counts, by using initial versus best algorithms for hospital emergency department chief complaint data, for selected BioSense data. Red line shows the initial algorithm (minimum SD = 0.2, 7-day baseline, count method, unstratifi ed baseline), and black line shows the best algorithm (minimum SD = 1.0, 28-day baseline, rate method, unstratifi ed baseline).