Female C57BL/6J mice were obtained from The Jackson Laboratory (Bar Harbor, ME) and infected when 4-6 weeks old. Mice were anesthetized with isoflourane and inoculated intranasally with 5610 4 plaque forming units (pfu) in 20 ml DMEM (Invitrogen). For intraperitoneal infections, 1610 3 pfu in 0.2 ml PBS were injected into the peritoneal cavity of mice. Lungs were harvested at 3, 5, or 7 dpi, homogenized with a tissue homogenizer for 1 minute at 24,000 rpm in 10 ml DMEM with 10% FBS, 100 U of penicillin per ml, and 100 mg of streptomycin per ml (Pen/Strep, Invitrogen). Spleens were harvested at 17 or 19 dpi. To identify any preformed infectious particles, half of each spleen was homogenized as above in 5 ml DMEM with 10% FBS, and Pen/Strep. The tissue homogenate was then assayed for viral particles by plaque assay on monolayers of NIH 3T3 cells overlaid with 1% agarose for 4 days. The cells were then fixed and stained with 0.04% methylene blue. Splenocytes were isolated from the other half of the spleen by dissociating the spleen and passing through a 40 mm cell strainer (BD-Falcon). Cells were then pelleted, resuspended in red blood cell lysis buffer (150 mM NH 4 Cl, 10 mM KHCO 3 , 0.1 mM EDTA), and incubated at room temperature for 5 minutes. Cells were again pelleted and resuspended in RPMI medium with 10%FBS and Pen/Strep before counting. Cervical lymph nodes were harvested at 10 dpi and isolated in the same manner as splenocytes, but without lysing the red blood cells.