In permissive cells, the first viral structures detectable by electron microscopy are the crescent-shaped forms ( Figure 3A ), consisting of a membrane with spicules on the convex surface [39] . These structures develop into non-infectious spherical immature viruses (IV) ( Figure 3B ) from which the intracellular mature virus (IMV) is formed by a series of maturation steps ( Figure 3C ). The IMV represents the majority of infectious progeny from each infected cell [17, 39, 55] . There are three possible mechanisms by which poxviruses are released from host cells depending on the strain of virus, cell type and the post-infection time [54] [55] [56] . They can be released by cytolysis, in which case IMV are released when the cell undergoes lysis as a result of CPE at the advanced stage of infection. It can also be released via virus-induced exocytosis. The third way of release is by budding, in which case IMV migrates out of the virus factory through the plasma membrane. Budding is shown to be the main exit route for APV [39] in contrast to the orthopoxviruses which exit by exocytosis of intracellular enveloped virus (IEV) [57] . These exit processes all result in the acquisition of an additional double membrane [39, 55] . In non-permissive African green monkey cells (CV-1, Vero) and in a human cell line (MRC-5), there is a blockade of the APV morphogenesis cycle. This occurs in steps following the formation of immature virus and is shown to be devoid of an alteration in early gene expression [47, 58, 59] , indicating that this blockade may not be associated with cell receptors.