Pathology of Viruses

[Keith Sturts]

Can anyone explain to a layman what the pathology mechanism is that
creates 'symptoms' of virus invasion?
 
In my limited pathology knowledge, I can think of two possibilities:
a) the virus disturbs the normal metabolism of the cell
    (perhaps this is two broad to define without delimiting)
b) the metabolic waste of virus presence and activity overloads the cell(s)
thus causing sick symptoms in the host.
 
It is suprisingly hard to find physiological explanations as to why viruses
effect animal life.  There is no shortage of symptomatic description,
however...
 
Thanks