Based on epidemiological considerations, it is postulated that the relatively high risk of MS in the industrialised nations is due to hygiene-related changes in the sequence of common infections, resulting in the emergence of patterns of immune reactivity that either cause, or fail to protect against, the development of MS. Further epidemiological studies are required to determine whether the timing of EBV infection is related to the risk of MS [20] . Based on studies of altered immune responses to certain infectious agents and evidence for the expression of HERVs in MS, epitopes that have a putative role, depending on the appropriate or inappropriate activity of immunoregulatory networks, in protection against or predisposition to MS have been delineated.