This paper argues that multiple sclerosis (MS) is the result of an autoimmune attack against components of the central nervous system (CNS). The effector cells involved in the pathogenic process are CNS‐autoreactive T cells present in the healthy immune system in a resting state. Upon activation, these cells cross the blood‐brain barrier and attack the CNS target tissue. Recent evidence indicates that autoimmune activation may happen in the intestine, following an interaction of bacterial components of the gut flora with local CNS autoreactive T cells. The consequences of this concept are discussed.