Until recently, it has been generally assumed that all human viruses are causative agents of diseases and should be regarded as harmful pathogenic agents that require control measures. In the early 1950s we began to doubt this view. In the course of experiments on virus isolations from feces of normal children, as well as in studies of isolates from animals and from tissue cultures, data accrued which suggested that some conditionally pathogenic and some completely nonpathogenic strains of enteroviruses may provide some benefit to their host by inhibition of pathogenic viruses and by activating nonspecific protective functions of the organism. The novel concept of beneficial viruses was proposed which suggested that the process of co-evolution of the host organism and its associated viral flora led to a specific interaction between them that was beneficial for both. This concept provides a potential approach to the nonspecific prevention of viral diseases by means of the interference between beneficial enteroviruses and pathogenic viruses belonging to different classes.