Cholera-causing bacteria are locked in an evolutionary arms race with a viral nemesis, according to a new genomic study. Researchers have found that, in the Ganges Delta, cholera bacteria rapidly gain and lose special armor that protects against attacks from the virus, known as bacteriophage ICP1. The study, published today in Nature, highlights that maintaining these antiviral defenses leads to lower disease severity of cholera in humans and reduced ability to spread outside the country for this bacterial strain.