Figure 2.

The Germline Hypothesis of Chronic Viral Infections. Once a chronic virus has been transmitted to a new host (indicated by the vertical lines), within-host rates of viral evolution can be extremely rapid, indicated by increasing evolutionary distance as time progresses. If there is heterogeneity in the rates of evolution among different within-host viral lineages, and if more slowly evolving lineages are more likely to be transmitted because they contain fewer transmission-reducing mutations, the rate of evolution of the virus at the between-host level will be slower than the rate of evolution measured at the within-host level (indicated by the red line). These slowly evolving lineages can be considered the viral germline.