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Infections with respiratory syncytial virus Long strain, associated with influenza virus, A/Beijing 353/89 (H3N2) strain, parainfluenza virus type 3, 739-2D strain, and adenovirus type 3, were experimentally induced in white mice, causing histological, histochemical and histoenzymatic lesions at the respiratory system level, the severity of which exceeded the one observed in the controls infected with a single virus. The pathomorphological changes made up an inflammatory, predominantly infiltrative, lymphohistiocytic, then exudative and alterative picture. The severest and most frequent lesion was the diffuse interstitial, often peribronchiolovascular, bronchopneumonia, which might involve large parenchyma areas. Another highly frequent pulmonary lesion was the thickening of interalveolar septa, due to stasis hyperemia, oedema and the predominantly lymphocytic cytoinfiltrate. At the level of the extrapulmonary airways, the lesion present in all experimental models was the denudation of epithelium cilia. In the viral association in which influenza virus was included, an alteration, the hyalinosis of tunica media of the vessels, as well as of the Reisseisen's muscle, was also observed, in addition to the cytoinfiltrate; when the association was achieved with parainfluenza virus type 3, many macrophages and erythrocytes and a few fibroblasts appeared in the cytoinfiltrate, the alteration being the same as in the former model; when the association contained adenovirus, there appeared necrosis, abundant lymphocytes and lysis of the Reisseisen's muscle in the bronchopulmonary block. The associated infections were demonstrated by the presence of homologous serum antibodies and by positive IF reactions in the pulmonary tissue.