a "happy" virus

[Eric Engelhard]

J. H. May wrote:

I was taught that viruses that destroy their hosts or cause them great
damage 
are indicative of primitive forms. If this is the case then surely
viruses 
would have evolved that benefited their host, possibly by improving
life 
expectancy and increasing gregarious activity for example, both
activities 
increasing likelihood of infection of others; wouldn't a happy host be
a 
better host?

Well:

Viral pathogenicity is not always a selective disadvantage. Look at
baculovirus pathology as an example: total tissue destruction
(liquification) of the host with release of up to a billion infectious
virions. And what about HIV: eventually lethal, but a long gestation
and with concurant infectivity. Replication is the ultimate goal and
there are many pathways, including high pathogenicity, towards that
goal.