Fig. 1 Morphology and structure of an orthopoxvirus virion. A: An electron micrograph of a thin section of orthopoxvirus infected cells. The structure of the virion reveals a condensed nucleoprotein organization of DNA. The core assumes a dumbbell shape surrounded by a core wall invaginated by large lateral bodies, which are in turn enclosed within a membrane to form intracellular mature virus (IMV). The circular and partially circular shapes are intermediate stages in virus assembly (immature particles). Magnification 120,000×. B: A schematic form of the virion.