Any approach towards elucidating the aetiology of an ill defined disease such as 'epidemic neuromyasthenia' has to be a comprehensive and wide-ranging one. Although viruses must be strong candidates, by reason of their ubiquity, this need not necessarily be the case and we have recently seen the onset of Legionnaires' disease as a new entity caused by a bacterium. We do not always recognize that a particular virus may affect the entire community and that the patient seen in hospital may represent only the tip of the submerged iceberg and that, in closed communities more of the iceberg will be seen. Silent viral epidemics are probably frequent and may only be recognized in retorspect. As an example the recent epidemic with adenovirus type 7 will be alluded to. Possible variations in virus and, to a lesser extent, in the host which could modify the course of an individual infection are discussed.