[Environmental factors: the contribution of infectious agents].

Multiple sclerosis (MS) is a multifactorial disease resulting from complex interactions between predisposing genetic and environmental factors. Among the many potential environmental risk factors, several common infectious agents such as Epstein-Barr virus (EBV), human herpesvirus-6 and Chlamydia pneumoniae(C. pneumoniae) have been causatively implicated in the onset of MS. However, with the exception of EBV, consistent data are yet to be obtained regarding the involvement of infectious agents. With respect to the Japanese population, we found that EBV infection is a risk factor for the subgroup of Japanese MS patients not harboring the HLA-DRB1*0405 allele, a known genetic risk factor for MS in this ethnic group. By contrast, bacterial infections such as Helicobacter pylori and C. pneumoniae are risk factors for Japanese neuromyelitis optica, especially in patients with anti-aquaporin 4 antibodies.