A distinct possibility is that other infections synergise with EBV in the aetiology of the disease and the timing of infections might be important. In this context, Chlamydia pneumoniae (CP) infection may play a special role [26] , as in a recent study on children with MS, CP-specific IgM antibody points to a high frequency of fresh, recent or reactivated infection with this pathogen at the onset of the disease [12] . A final key point is the demonstration that slight, though statistically significant, elevated levels of antibody to certain common infectious agents other than EBV and CP occur in children and adults with MS compared with age matched control subjects [12, 15, 27] . This finding does not, however, imply a direct causative relation of any infection to MS as it could reflect a more general dysregulation of immune function as a cause or consequence of the development of the disease. It is indeed likely that the elevated antibody concentrations have no significance per se for the development of MS but reflect a shift in patterns of immune reactivity away from a protection and towards enhancement of the risk of disease. Nevertheless, studies on these MS-associated infectious agents could lead to the identification of specific antigenic determinants involved in the generation and maintenance of immune dysregulation.