Figure. The prevalence of illness pyramid. Passive surveillance data represent only the tip of the iceberg. For a bacterial infection to be included in the passive surveillance system, it must pass through the following steps: a person becomes ill with a diarrheal disease, the patient must go to a doctor, the doctor must order a bacterial stool culture, the assigned microbiology laboratory must culture for this organism and report the infection to the state health department, and the state health department in turn must report the infection to CDC. This passive surveillance system is the means by which the number of cases of foodborne illness is currently determined at CDC; if any step does not occur, foodborne illness is not reported. FoodNet is designed to collect information along each step of this pyramid.