In the previous section, we discussed strategies by which flies can be used to screen for pathogen virulence factors. In this final section, we consider the advantages of Drosophila as a model for analyzing virulence factor function, and for identifying the host proteins and pathways that they target (e.g. Fig. 2C ). Although cellbased expression systems and biochemical experiments performed with purified virulence factors can be invaluable for establishing mechanism of action, they do not necessarily predict how such factors will act in an infected organism -either systemically or in selected tissues, in which cell-autonomous and non-cellautonomous processes might be important. Model systems such as flies and worms are ideal for this level of analysis owing to the great variety of genetic tools available to tease apart the effects that such factors might have on specific host pathways and biological processes. Although flies and worms are only distantly related to humans, many virulence factors target host proteins and pathways that are among the most conserved in eukaryotes -thus, studying the effect of pathogens in these organisms is often highly relevant to human disease. In addition, as discussed below, studies in model organisms also enable examination of the combinatorial effect of two or more virulence factors, which is more challenging in intact mammals. Finally, we highlight in Box 1 how studying the effect of toxins can shed light on basic cellular processes.