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Gnutella-like Peer-to-Peer Networks, due to their extreme connectivity fluctuations, are highly robust against random failures. However, such error tolerance comes at a high price of attack survivability. In this paper, to enhance such attack survivability, we propose a new formulation used for defense against deliberately attacks based on two leading concepts: cost and load. The cost measures how expensive it is to cure an attacked or infected node, and the load measures how important a link between two nodes is when propagating attacks or updating immunization information reversely. The combination of these factors leads us to introduce the concept of optimal targeted immunization, which formalizes the ideas of minimizing the risk of epidemic outbreaks in these networks. Using this analysis framework, we then devise a novel efficient targeted immunization scheme. The simulation results under a realistic Gnutella network show that our immunization scheme outperforms other existing methods, producing an arresting increase of the network attack tolerance at a lower price of eliminating malicious attacks.