The popularity of peer-to-peer (P2P) networks makes them an attractive target to the creators of viruses and other malicious code. Indeed, recently a number of viruses designed specifically to spread via P2P networks have emerged. In this paper we present a model which predicts how a P2P-based virus propagates through a network. This model is a modified version of the S-E-I (Susceptible-Exposed-Infected) model from the field of epidemiology. Our model classifies each peer as falling into one of

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... three categories based on the number of infected files it is sharing. We derive differential equations which comprise the deterministic model and examine the expected behaviour of the P2P network as predicted by these equations.