Kinetic observation of lung lesions of SD/09-virus-infected mice is shown in Figure 2 . On day 4 p.i., lung lesions were characterized by dropout of mucous epithelium and inflammatory cells adhering to the bronchiolar surface ( Figure 2C , D). On day 6 p.i., severe edema could be seen around blood vessels ( Figure 2E ); interstitial pneumonia was also observed that showed interstitial edema and thickening of the alveolar walls; and the alveolar lumen was flooded with detached alveolar cells, erythrocytes, and inflammatory cells ( Figure 2E , F). On day 8 p.i., the virus caused more severe interstitial pneumonia and peribronchiolitis, characterized by edema and extensive of lymphocytes, neutrophils and plasma cells around the area of bronchiolitis ( Figure 2G , H). Lesions in the lungs of infected mice were still severe on day 14 p.i., with extensive alveolar collapse, and remaining alveoli were filled with fibrin, desquamated alveolar cells, and inflammatory cells. Lymphocytes and alveolar macrophages were the predominant inflammatory cells observed at high magnification ( Figure 2J ). Masson's stain revealed that alveolar walls and spaces were filled with collagen fibers ( Figure 3A , B), indicating that proliferative fibroblastic lesions may develop. In comparison, lungs from mockinfected control mice had no apparent histological changes ( Mice were inoculated i.n. with 10 2.5 pfu SD/09 viruses; tissues were collected at indicated times p.i. and viruses were titrated in MDCK cells. Body weight, lung wet and dry weight were determined and recorded. The lung wet weight:body weight ratio and lung wet:dry weight ratio were calculated and used as an indicator of lung edema. *p,0.05, **p,0.01, ***p,0.001, comparison between ratios obtained from the virusinfected and control groups. Bars represent means 6 SD of data from three mice. doi:10.1371/journal.pone.0029347.g001 Immunohistochemistry revealed viral antigens in the epithelial cells of the bronchioles ( Figure 3D ), terminal bronchioles, and alveolar epithelial cells ( Figure 3E ). These data indicated that SD/ 09 virus could infect the epithelia of the lower airway and cause viral pneumonia in mice.