An environmental stimulus to cause multiple sclerosis (MS) has been debated for years – usually viruses or bacteria. However, a new paper by Kerstin Berer and colleagues at the Max Planck Institute of Neurobiology in Martinsreid in Germany points to the commensal bacteria in the GI tract. Using the EAE mouse model of MS, they show that gut bacteria are essential for triggering the auto-immune attack on the central nervous system. That is, germ-free mice induced to have auto-immune disease remain healthy until they are colonized with commensal bacteria. The results are quite dramatic. This adds to the evidence of a role for gut microbiota in inflammatory bowel diseases, rheumatoid arthritis and type 1 diabetes. Somehow the commensals are seen as foreign under certain circumstances?