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

The probability of evolutionary emergence. We first focus on the scenario of a single individual in the host population getting infected by the wild-type strain. We assume that the mean number of individuals infected by this individual (the reproductive number R w of the wild-type) is less than one. Hence, in the absence of mutations, there is no chance of a major outbreak [92] . However, if the wild-type strain produces a mutant strain whose reproductive number R m is greater than one, there is a chance for a major outbreak. The mutant strain might have a higher reproductive number than the wild-type strain because it replicates more rapidly within the host or because it transmits more effectively to new hosts (or both). We define these within-host and between-host selective advantages as s = r m − r w and τ = log(b m ) − log(b w ), respectively.