Shigella surveillance in a large metropolitan area: assessment of a passive reporting system.

A passive disease report card (DRC) surveillance system failed to detect an epidemic of diarrheal disease caused by a newly identified drug-resistant strain of Shigella sonnei. The DRC system inaccurately described both the population at risk and the geographic location of cases. Specific limitations of the DRC system, including problems of underreporting and ascertainment bias, were complicated by the absence of timely data analysis and feedback to providers.