Population structure, such as the number of young children and elderly persons, may account for some of this difference, because the incidence of infl uenza infection is highest in these age groups. As with ILI, the case defi nitions used may have affected the data or the use of different laboratory confi rmation tests for infl uenza infection may have resulted in considerable variation in disease rates between studies; the study in Thailand used RT-PCR for laboratory confi rmation. Although the rates of infl uenzaassociated pneumonia were different in the refugee camp, the proportions of pneumonia cases associated with infl uenza were similar (23% vs. 18%).