Emerging Infectious Diseases • www.cdc.gov/eid • Vol. 16 2009 were detected. During May-August, the incidence of LRTI and URTI in cases captured by the passive surveillance system was higher each month in 2009 than in 2008. The rates of URTI were similar in September and October of both years, whereas the LRTI rate was higher in October 2008 than in October 2009. Pandemic (H1N1) 2009 did not clearly increase in case-patients with ARI after its fi rst detection in the camp in August 2009. However, surveillance did not capture mild infections that did not result in visits to the outpatient department. The occurrence of most infl uenza A infections in patients who had pneumonia most likely refl ects a sampling bias, although infl uenza is a generally underrecognized cause of pneumonia in the tropics (24) . ILI is not a routinely used diagnosis for the clinic staff at Maela, so most of the ILI case-patients likely were not interviewed and sampled. However, when infl uenza A was identifi ed, pandemic (H1N1) 2009 case-patients were less likely than seasonal infl uenza case-patients to have been hospitalized. This information suggests that, in this population, illness caused by pandemic (H1N1) 2009 was no more severe than illness associated with seasonal infl uenza A. Several confounding factors, unrelated to the innate pathogenicity of the viruses, may account for this fi nding: 1) the timing of the modifi cation of case defi nitions in relation to the appearance of pandemic (H1N1) 2009; 2) differences in age distribution; and 3) presence of underlying illnesses in the patient groups. Data regarding underlying medical conditions were not collected as part of this surveillance so the effect of other conditions cannot be assessed. To prevent spread of infection, public health systems may request persons with ILI to self-quarantine, which might result in underestimation of the number of cases identifi ed in clinic-or hospital-based surveillance systems. During the 2009 infl uenza season, announcements regarding infl uenza and the need for good hygiene were made on the Maela public address system; healthcare workers reinforced these messages by home visits. Whether this intervention had any effect on healthseeking behavior remains unclear. An infl uenza triage system was in operation at the hospital, but our surveillance staff had access to patients seen and treated in this area.