Fig. 1.
Pathogens rapidly increase fitness during serial passage. Serial passage of virus through BALB/cbb (dark gray boxplots), BALB/cdd (light gray boxplots), and BALB/ckk (white boxplots) host genotypes produces virus stocks with significantly higher fitness. Data above are shown on a natural log scale. Both proviral load measures (A) and infectious particle counts (B) are significantly higher in animals infected with passaged vs. unpassaged viruses across all three MHC-congenic mouse strains.