evolving beasties and G+C

[the kipster]

out of interest, why does G=C content vary so much for mammalian
viruses which on the whole need to use a cell with a GC content of
47%.  ? The most peculiar are members of the herpesviruses; HSV (67%)
and VZV (47%) both of which grow largely in the same cell types
(dermis, skin and neurons)   Is this a result of pressure from
unusual features of the viral polymerase, or is it another mechanism? 
I have never been able to answer this to my inquisitive students   ...

What happens for GC content in bacteria and phage beasties?