Most viruses take over the cell's replicative machinery. The cell dies after 1-2 days when it contains masses of virus particles and is unable to repair itself. The masses of virus particles appear as inclusion bodies by staining.

A common problem in diagnosis is that the tissue homogenate is toxic . If the toxicity is of viral origin the medium above the dying cells can usually be diluted at least one millionfold and then kill new cell cultures.