The observed discrepancy between government statistics obtained by passive surveillance and shigellosis rates detected by active surveillance highlights a fundamental problem in the surveillance methods used. In studies using passive surveillance, cases are detected when patients present to a health-care provider participating in surveillance. In active surveillance studies, healthcare providers or other study staff visit each member of the study population at regular intervals and enquire about disease episodes occurring since the last visit. Passive surveillance should ideally detect all treated episodes. But there is always a risk that patients may seek treatment from health care providers who are not participating in surveillance activities. Thus, rates estimated by passive surveillance may underestimate true incidence rates.