For the disease to be maintained in the population, this newly infected individual has to contribute to the infection of one or more susceptible individuals and so on. Population density and probability of effective contact are very much dependent on spatial characteristics in which a population evolves. Diseases in livestock and poultry may be transmitted easier 2 miles down a road in a valley than to a farm half a mile away across a mountain ridge. There is no clear distinction in the definitions between emergence of novel disease agents, recognition of disease agents that had been present in a population but were not recognized because they caused minimal problems or could not be recognized because of diagnostic limitations, and reemergence of disease agents. The environment may increase host susceptibility through environmental stressors.
Environmental stressors can contribute to disease in a resistant host.
The simplest form of the model maintains p and d constant. The number of susceptibles relative to the number of infectious individuals at some point during a propagating disease outbreak decreases to a point where effective contact is not likely anymore. By that time point R 0 is below 1 and the disease is halted in its rogression in the population.