Due to the nature of the interaction between a host and a parasite many host populations will develop some form of immunity.
The development of host population immunity causes either a decline in the parasite population or a genetic mutation in the parasite population to overcome the immunity.
This dynamic tends to form a cycle in the parasite population where there are periods of population increases and periods of population decreases.
Maja Jurc, University of Ljubljana, www.forestryimages.org
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