Using bacteria-killing viruses to treat bacterial infections, or phage therapy, can be a good alternative to antibiotics in some situations when there are no effective antibiotics for a particular infection. But bacteria can evolve resistance to phages as well as antibiotics, often with little cost to their fitness. In this study, a phage not only could treat an infection by attacking the bacteria, but the bacterial hosts that do evolve resistance to the phage do so by getting rid of certain structures that help them to cause more serious infection. Thus, therapy with this phage may both reduce the bacterial load and also make those remaining less virulent.