Recent clinical and experimental studies indicate that multiple sclerosis (MS) develops as consequence of a failed interplay between genetic and environmental factors. An ever‑growing number of risk genes have been recognised that support an autoimmune response against the body’s own brain matter. Together, these increase susceptibility to MS without actually triggering the disease. Recent experimental observations indicate that the actual trigger of the autoimmune attack is provided by an interaction of brain-specific immune cells with microbial organisms. Unexpectedly, these microbes are not necessarily responsible for infections, but are components of the healthy commensal gut flora – the intestinal microbiota. This concept opens the way for new therapeutic approaches involving modulation of the microbiota by dietary or antibiotic regimens.