A study has revealed an insidious aspect of the gut protozoan Blastocystis, which is largely thought to be a harmless commensal organism. A Singaporean subtype of Blastocystis, ST7, caused the destruction of ‘good’ bacteria in the gut through activating reactive oxidative species. ST7 also caused damage to the gut epithelial lining. This discovery has implications for inflammatory bowel disease, irritable bowel syndrome, gastrointestinal cancers, and fecal microbiota transplantation.