Figure 1

Genealogical trees derived from hemagglutinin sequences of influenza A (H3N2) and measles virus. The influenza tree, sampled between 1968 and 2008, appears long and spindly with a distinct lack of deep branches. It only takes a few years for contemporaneous strains to find a common ancestor. The measles tree, sampled between 1979 and 2009, looks very different, harboring many deep branches. It takes approximately 50 years for contemporaneous measles strains to find a common ancestor.