Histopathological lesion of the lung of the pigs infected with H1N1, H1N2 or H3N2 and non-infected pigs at post-inoculation days (PID) 2 and 14. The lung section of pig infected with H3N2 shows diffuse necrosis of bronchiolar epithelium and infiltration of inflammatory cells composed of mostly neutrophils and mononuclear cells around bronchioles (C), however most lesions were restored in the pigs at PID 14 (G). The pig infected with H1N1 shows less inflammation around bronchioles, but exhibits hemorrhagic lesions at PID 2 (A), whereas chronic inflammation with lymphocytic infiltration and epithelial proliferation is observed in H1N1-infected pig at PID 14 (E). The pig infected with H1N2 shows mild interstitial pneumonia lesion at PID 2 (B), but the lesion was restored at PID 14 (F). Control pigs show no histopathologic lesion at PID 2 (D) and at PID 14 (H).