Population ecology, genetics and disease emergence Of particular relevance to disease emergence is the explanation provided by theoretical population biology, already mentioned, of how host (including human) population size determines whether or not a pathogen can persist in a population. The accumulated findings demonstrate thresholds exist, depending on the type of pathogen and host population, below which a pathogen cannot be sustained. Considered in light of the exponentially increasing size of human and domestic host and vector populations in the world, the breaching of thresholds of pathogen persistence can explain much of the increase in emerging infectious diseases.