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. 2011 Sep 22;6(9):e24257. doi: 10.1371/journal.pone.0024257

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Rossi et al.

This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are properly credited.

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Individuals are classified into 3 states: susceptible (SU), immune (IM), infected (INF). Transitions are possible between all states except from INF to SU (fixed at zero).