Figure 3. Lung of prairie dog infected with monkeypox virus, showing abundant intraalveolar mixed inflammatory infiltrate and necrosis (A: hematoxylin and eosin stain, 50X original magnification). Orthopox viral antigens are abundant in the cytoplasm of the bronchiolar epithelium (D: immunohistochemical assay anti–variola virus antibody, 100X original magnification). Macrophages, fibroblasts, and alveolar epithelial cells, as well as necrotic debris demonstrate orthopox viral antigens in pneumonic areas of the lung (C: immunohistochemical stain anti–variola virus antibody, 100X original magnification). Accumulation of intracellular mature virions (arrow) in bronchial epithelial cell (arrowhead pointing to cilia) (B: transmission electron microscopy, 2,400X original magnification).