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During the time that synthesis of viral components is occurring in the infected cell, the cell undergoes characteristic biochemical and morphological changes. Progression of these changes is most readily observed in cell culture, where infection of cells is more easily synchronized and where the cells can be observed and sampled frequently during the course of infection. Morphological changes in cells caused by viral infection are called cytopathic effects (CPE); the responsible virus is said to be cytopathogenic. The degree of visible damage to cells caused by viral infection varies with type of virus, type of host cells, multiplicity of infection (MOI) and other factors. Some viruses cause very little or no CPE in cells of their natural host. Their presence can be detected visually only by hemadsorption or interference, in which infected cell cultures showing no CPE inhibit the replication of another virus subsequently introduced into the cultures, or in situ by viral antigen or nucleic acid detection. On the other hand, some viruses cause a complete and rapid destruction of the cell monolayer after infection. The microscopic appearance of the CPE caused by some of these cytocidal viruses may be sufficiently characteristic to allow provisional identification of an unknown virus.

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