Pathogenic bacteria sometimes wreak havoc thanks to viral invaders called bacteriophages. Some phages infect bacteria and incorporate their DNA, as well as genes for toxins and other virulence factors, into the genomes of the microbes. In fact, the genes for the cholera toxin came from a phage that infected the ancestor of Vibrio cholerae. Such phages lie dormant and then reawaken and burst out of bacteria to infect other bacteria nearby, spreading the virulence genes they carry in the process. A new study reports that inflammation caused when the immune system attacks bacteria triggers this viral spread between microbes (Science 2017, DOI: 10.1126/science.aaf8451). The findings suggest that vaccinations and other treatments that avoid inflammation could slow the evolution of pathogens. The researchers led by Médéric Diard and Wolf-Dietrich Hardt of ETH Zurich studied mice infected with two strains of Salmonella enterica Typhimurium: a donor that contained a phage and a recipient that did not. After three days in the mice, 58% of the recipient microbes had been infected by the phage. But when the team used Salmonella engineered to not trigger inflammation, less than 0.01% of the recipient microbes carried the phage. Also, when the team infected mice vaccinated against Salmonella, the researchers observed less gut inflammation and, as a result, less phage transfer.