“…Many agents have been found in association with exacerbations, but their relative importance has varied greatly in different studies. In particular, viruses have been identified with very variable frequency in several careful surveys (Somerville, 1963;Carilli et al, 1964;Eadie et al, 1966;Ross et al, 1966;Stenhouse, 1967;McNamara et al, 1969;Fisher et al, 1969). The paucity of viruses in some surveys is hard to reconcile with the common observation made in general practice that patients often have exacerbations of bronchitis when other members of the family are suffering mild, and presumably mainly viral, infection of the respiratory tract.…”