Trouble with figuring out functionality of viral oncogenes

[Spork]

Hi folks, I have an embarrassing hole in my knowledge I am trying to fill. I've been reading up on Erb2 I started thinking about viral oncogenes. What I am having trouble finding out is not the relation of the oncogene in the host cell, but what the functionality of various viral oncogenes have for the virus itself. What is the original functionality of the gene from the virus' perspective.  Almost all the literature searches that I try just talk about the cancer aspects. 

Also, if anyone has a helpful link to this or related topics, I would be most grateful.