We are studying the complex genome evolution of bacteriophages and their coevolution with bacteria. The amount of genetic variation among phages is greater than within any other group of organisms. The genomes of many phages are made up of interchangeable modules which can be horizontally transferred between genomes. Phages can also pick up genes from one bacterium and deliver them to another, which makes them a very important factor in the evolution of bacteria. Alike other host-parasite systems, phages have evolved different levels of virulence. Some phages are extremely virulent and instantly kill their host while others have the ability to integrate into the bacterial genome and establish a mutualistic relationship with their host.