∞ generated and posted on 2016.03.03 ∞
Protection of populations from pathogens due to acquired resistance by only a portion of the population.
For example, through immunization the number of susceptible individuals within a population could be reduced to, say, 25%. If each infected individual in a fully susceptible population were to infect two others, then by reducing susceptibility by 75% the number of new infected individuals per infected individual could be lowered to less than one, resulting in a dying off an epidemic.
This idea of number of successful transmissions from each infected individual is described as pathogen or parasite's reproductive number.