Strangles, caused by the bacteria Streptococcus equi, is the most frequently diagnosed infectious disease of horses, with 600 outbreaks estimated to occur in the United Kingdom each year. Streptococcus equi invades the lymph nodes of head and neck of horses, causing them to swell and form abscesses that can, in around 2% of cases, literally strangle the horse to death. Some of the horses that recover from strangles remain persistently infected. These apparently healthy animals shed bacteria into the environment and spread the disease to other horses that they come into contact with. Using standard diagnostic testing, the Streptococcus equi strains look almost identical. But by carefully examining the DNA of the bacteria, the team were able to track different variants as they spread across the world.