B6 mice gradually lose their natural resistance to mousepox as they age. (A) B6 mice of the indicated ages were infected with WT ECTV in the footpad and survival was determined. 2–3 mo (n = 50) versus 6–9 mo (n = 44), P = 0.0205; 2–3 mo versus 10–14 mo (n = 19), P = 0.0001; 2–3 mo versus 14–18 mo (n = 47), P < 0.0001. Animals of both sexes were used. (B) Young (Y) and aged (A) B6 mice were infected with WT ECTV, euthanized 7 dpi, and the number of live cells/spleen was determined by trypan blue exclusion. Results for uninfected controls (U) are shown for comparison. Data correspond to the mean ± SD of pooled organs of two to three mice per group from three individual experiments. (C) Young and aged B6 mice were infected with WT ECTV and euthanized 7 dpi, and virus titers in spleens and livers were determined by plaque assay. Data correspond to the mean ± SD of pooled organs of two to three mice per group from three individual experiments.