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Training Manual on Wildlife Diseases and Surveillance
pathogen transmission and recovery of the infected individuals. Eventually, transmission will
cease and the pathogen will die out, will be eradicated.
The value of R for a particular pathogen and host species is not always the same. The
value of R changes with the characteristics of the environment and of the host population, and it
also can change during the course of a disease occurrence event. For example, in a population in
which there is no significant immigration or emigration of host animals, or birth of new,
susceptible individuals during the course of a disease event, the value of R will become smaller
and smaller as an ever larger proportion of the population consists of individuals which have
survived infection and are immune to further infection.
In such a situation, the pathogen eventually will die out when the last infected individual
either dies or recovers from infection. All animals in the population now are immune. How long
it takes for a pathogen to die out depends on the value of R. If R is only slightly less than 1, a
pathogen may remain in a population for a long time - for months, years, or decades, depending
on the situation. If R is much less than one, then the pathogen may die out quickly - in weeks or
months perhaps. On the other hand, if new susceptible individuals are added to the population
at a sufficient rate, R may never become less than 1, and the pathogen may persist forever in that
population. Thus, birth rates, death rates, immigration and emigration rates can have a very
large effect on the value of R.
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