Methods: During October 1, 2003--January 31, 2004, syndromic surveillance signals generated for detecting clusters of influenza-like illness (ILI) were compared with ILI activity identified through a sentinel provider system and with reports of laboratory-confirmed influenza.

Although syndromic surveillance was developed for early detection of a large-scale release of a biologic agent, current surveillance goals reach beyond terrorism preparedness. Medical-provider reporting remains critical for identifying unusual disease clusters or sentinel cases.

Epidemiologists use several types of surveillance to monitor diseases, such as syndromic surveillance, which tracks symptoms rather than diagnosed diseases. Meanwhile, sentinel surveillance monitors specific populations, while passive surveillance relies on individuals filing reports voluntarily.

Syndromic surveillance: an active or passive system that uses case definitions that are based entirely on clinical features without any clinical or laboratory diagnosis (for example, collecting the number of cases of diarrhea rather than cases of cholera, or "rash illness" rather than measles).