(Science: epidemiology) 1. Basic reproductive ratio , Ro , a dimensionless parameter which encapsulates the biological details of different transmission mechanisms. For microparasites, Ro , is defined as the average number of secondary cases of infection to which one primary case gives rise throughout its infectious period if introduced into a defined population consisting solely of susceptible individuals. For macroparasites, Ro , is the average number of female offspring (or just offspring in the case of hermaphroditic species) produced throughout the lifetime of a mature female parasite, which themselves achieve reproductive maturity in the absence of density-dependent constraints on the parasite establishment, survival or reproduction.
2. Effective reproductive ratio , R , The number of secondary cases (microparasites) or female offspring (macroparasites) produced in a host population not consisting entirely of susceptible individuals (microparasites) or within which density dependent constraints limit parasite population growth (macroparasites). Under [[cond 40c itions]] of stable endemic infection, R=1.