Phages are viruses that infect bacteria and can also be used to treat human infections. However, as with antibiotics, bacteria can readily evolve resistance to phage attack, highlighting a key limitation to the use of phages as therapeutics. Now, researchers have shown that the naturally occurring phage A1-1 kills Shigella flexneri, a major cause of dysentery in sub-Saharan Africa and southern Asia and selects for phage-resistant mutants with reduced virulence.