Figure 1

Survival ratios of mice following intranasal infection with different doses of influenza A/PR/8/34 (H1N1) viruses. Groups of five mice were infected intanasally with 5 × 105 (*), 5 × 104 (◇), 5 × 103 (♦), 5 × 102 (┼) or 5 × 101 (△) p.f.u. A/PR/8/34 (H1N1) viruses and were observed consecutively for 14 d.