The highly virulent A/Vietnam/1203/2004 (H5N1) virus, isolated from a human with a fatal case of influenza [19] , was used in this study. The virus stock was propagated in the allantoic cavities of 10-day-old embryonated hens' eggs following incubation at 37uC for 24 to 29 hours. Allantoic fluid from multiple eggs was pooled, clarified by centrifugation, divided into aliquots, and stored at 270uC. The 50% egg infectious dose (EID 50 ) titers were determined by serial titration of viruses in eggs and calculated by the Reed and Muench method [27] . The 50% lethal dose (LD 50 ) was determined as previously described [19] . All research with HPAI virus was conducted under Biosafety Level 3 containment, including enhancements required by the U.S. Department of Agriculture and the Select Agent Program [28] .