A virus may kill an infected cell creating characteristic cytopathic damage or may replicate in cells without any visible effect. The type of changes induced by a virus can be significant for the type of virus and can be observed by the light microscope. The cytopathic effect (CPE) can be focal, diffuse through the cell monolayer, or at the edge of the culture. Cells may appear to be rounded or enlarged, growing in grapelike clusters, indicating adenovirus or herpes simplex virus. Influenza and mumps cause cells to fuse together, creating syncytia that detach from the surface. Cells may fuse and form multinucleated giant cells with granular cytoplasm typical of measles infection. Vaccinia and poxviruses create foci of fused cells, whereas picornaviruses induce proliferation of membranes in the cytoplasm and shrinkage of the nuclei (pyknosis). However, many viruses such as members of Bunyaviridae, Arenaviridae, and Retroviridae fail to produce obvious CPE and can replicate in culture without any noticeable change.
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