where the function V (x) is the infectious net-reproductive function. It is an ''infectious age''-dependent function that defines the rate at which an infected host generates secondary infections in the time interval following its initial infection (File S1 Section i). This theory has been extended to address competition between emergent pathogen strains using basic reproduction number as the metric for competitive dominance [16] . Selective advantage,s s, in the case of competing strains is now given bỹ