“…One method is to compare children suffering from respiratory disease with those affected by nonrespiratory conditions or without any evidence of illness Holzel et al, 1965). Other methods include the study of the aetiology of outbreaks (Carmichael, McGuckin, and Gardner, 1968), the examination of the effect of introducing viruses of doubtful pathogenic significance into human volunteers (Jackson et al, 1963), and the investigation of closed communities of children into which, inadvertently, a virus may be introduced (Wolontis, Tunevall, and Sterner, 1967).…”