A strength of this study is its use of a cohort of case-patients particularly well-suited for investigation of potential host genetic risk factors-these case-patients died with active infl uenza infections, yet were predominantly children and young adults without severe preexisting medical conditions. In such a group, other factors associated with severe infl uenza are less likely to obscure possible genetic associations. An additional strength was access to postmortem lung tissue for immunohistochemistry and/or RT-PCR confi rmation of infl uenza infection.