Question

What happens when a cell transcribes a viral gene?

Answer 1

hmmm...
A lot of agony, and possibly mishap I suppose...

Explanation:

Many cells, especially prokaryotes, have defence mechanisms agains viral DNA ( think of Endonucleases).

If transcription HAS taken place however, that means that the viral Gene is seen as the host's own. and the transcript will be processed by the host's machinery as such: processing into mature mRNA, and ultimately translation into Protein (Structural protein for capsid, or viral enzyme).

Note: in the case of RNA viruses like the Hepatitis C-virus it actually can be seen as their own, as the viral (ss)RNA will be coded back into DNA by the enzyme Reverse Transcriptase...