The outcome of an infectious process depends on a very complex set of interactions with the immune system. A successful pathogen is usually one that has developed mechanisms that avoid fast elimination by an immunocompetent host. These mechanisms allow the in-fectious agent to replicate, cause disease, and spread to other individuals before the immune response is induced. The immune response, on the other hand, is a powerful weapon that, once set in motion, may destroy friendly targets. Thus, the therapeutic strategies in infec-tious disease have to consider all these questions, such as the particular survival strategy of the infectious agents, the effects of the infection on the immune system, and the possibility that the immune response may be more of a problem that the infection itself.