Histopathology and immunohistochemistry (IHC) analysis of infected mice. (a) Distribution of H7N9, H5N1, and H1N1 viruses in the tissues of infected mice as determined by IHC. Representative viral antigen distribution in tissues at 3 dpi is shown. Viral antigens are denoted with solid arrows (400× magnification). (b) Hematoxylin and eosin stain (HE) staining of lung tissues from infected mice (100× magnification).