Due to relative simplicity of the fly and availability of large amounts of genetic tools, Drosophila melanogaster has proven to be an excellent model to study the basic principles of innate immunity. This is illustrated by the discovery of the Toll-like receptor functions in pathogen sensing, recognised by the 2011 Nobel Prize in Physiology and Medicine awarded to Jules Hoffmann. Drosophila can also be used as an in vivo, genetically tractable model, to analyse various aspects of host-pathogen interactions including virulence factor mechanisms of action.