Peer-to-Peer (P2P) systems have become an important application in the Internet for information sharing. However, recent studies showed that pollution in P2P file sharing systems is very serious. Although some measurements have been done to estimate the pollution in P2P networks, how a polluted file is propagated among nodes and why the pollution is serious are not studied. This paper proposes a model to capture the major features of the pollution. The model describes the propagation of a polluted file and the competition among the polluted version and the good version of a file in a P2P network. The model shows that correct user strategies based on publisher's reputation can effectively stop pollution at the expense of some initial propagation.