During the synthesis of viral components within an infected cell, the cell undergoes characteristic biochemical and morphological changes. Visible morphological changes in cells caused by viral infection are called cytopathic effects (CPE); the responsible virus is said to be cytopathogenic. The degree and types of visible damage to cells caused by viral infection vary greatly. Some viruses cause little or no CPE; others can cause total destruction of a 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.

Recognizing CPE is an important diagnostic tool. Several types of CPE are distinguishable in living cultures, but fixation and staining of the cells is necessary to view manifestations such as inclusion bodies and syncytia. Syncytia are enlarged cells with four or more nuclei resulting from the fusion of the plasma membranes of four or more cells. Inclusion bodies are areas of altered staining due to accumulation of viral nucleic acids or proteins during viral assembly or due to the viral scarring of the cell.

Figure 1 shows Giemsa-stained bovine fetal spleen cells 48 hours post infection with bovine viral diarrhea virus, at a total magnification of 200x. CPE can be seen. In the magnified, labeled version of the micrograph, arrows point to CPE in the form of vacuolization (formation of cytoplasmic vacuoles due to viral infection). No inclusion bodies are seen with this virus.

Figure 2 shows noninfected BFS cells at 200x.