We present a simple model of population dynamics in the presence of an infection. The model is based on discrete-time equations for sane and infected populations in interaction and correctly describes the dynamics of the epidemic. We find that for some choices of the parameters, the model can possess conserved quantities. We also propose an ultra-discrete, cellular-automaton, version of the model which despite its extremely simple structure still captures the essence of the epidemic dynamics. (C) 2003 Published by Elsevier B.V. [References: 9]
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