The search for the aetiology of multiple sclerosis (MS) has led to many theories over the years. It has become clear, however, that MS is complex and multifactorial, involving many interlacing mechanisms, many of which appear to be playing a role in the causation and evolution of the disease. In the large part for epidemiological reasons, an infectious aetiology has always been a prime suspect in this search. Although many agents have been examined over the years, the Epstein-Barr virus (EBV) has re-emerged as a candidate pathogen, based to a large degree on sero-epidemiological evidence, and on the knowledge of the effects this virus is capable of producing in other situations.