An aberrant proportion of out-of-catchment zip code ILI encounters signaled an out-of-control process (or alert) on July 23, 2009, 2 days later than the beginning of the influenza outbreak at the facility (Fig. 1 ) on the date when the majority of affected participants were evaluated for flu symptoms. (The alert on July 26, 2009, was a part of this same outbreak.) Using this algorithm, there were two other days in the 7-month period during which the chart signaled that the process was out-of-control: March 21, 2009, and September 6, 2009 . Investigations are being conducted to determine the nature of these other signals.