A hypothesis which relates MS to the 'biography' of the immune system On the basis of the available epidemiological evidence it may be postulated that MS is dependent on an infection with EBV which, owing to hygiene-related factors, occurs later than usual in life. Under such circumstances, the EBV infection might encounter patterns of immune responsiveness generated by prior infections with certain other microorganisms. In this paper we suggest a scenario in which the sequence of certain common infections results in immune dysregulation favouring the onset of MS and in the following sections this hypothesis is elaborated.