Cross-scale dynamics and the evolutionary emergence of infectious diseases

Sebastian J. Schreiber; Ruian Ke; Claude Loverdo; Miran Park; Priyanna Ahsan; James O. Lloyd-Smith

First, the size of the minor outbreak produced by the wild type determines the number of opportunities for the mutant strain to appear within a host. The average size of this minor outbreak equals S w = 1 1−Rw , as noted by earlier studies [12, 13] . If the wild strain is badly maladapted, then it is expected not to spread to multiple individuals (e.g. if R w < 1/2, then 1 1−Rw < 2) and opportunities for transmission of mutant virions are very limited. Alternatively, if the wild strain is only slightly maladapted to the new host, then, even without any mutations, the pathogen is expected to spread to many individuals (e.g. if R w = 0.95, then 1 1−Rw = 20), thereby providing greater opportunities for evolutionary emergence. Our analysis implies that higher contact rates, within-host viral growth rates, viral transmissibility, and maximal viral loads (for long infectious periods) facilitate these larger reproductive values.