Fig. 3.

Pathogen adaptation results in a significantly more virulent infection. (A) Measures of virulence were compared between the same groups of animals used in the experiments summarized in Fig. S1. Based on an analysis of pooled data, infection with postpassage vs. unpassaged virus stocks was associated with a highly significant increase in spleen weight. Data shown on natural log scale. (B) Typical size difference between the spleens and livers of BALB/c animals infected with either unpassaged (Upper) or postpassage (Lower) virus stocks.