SH2 domains serve as the prototype for a growing family of protein-interaction modules, characteristic of polypeptides involved in transmitting signals from external and internal cues. The specific interactions of proteins with one another, and with other cellular components such as phospholipids and nucleic acids, provide a very general device to organize cellular behavior. We discuss the idea that rewiring of the cell's interaction network by pathogenic microorganisms and mutant cellular proteins contributes to dysregulation of cell signaling and thus to disease.