I think it makes more sense if you think of a virus as a software update for a genome. The question is not how a virus made it here--fashioning a delivery mechanism should be easy for a civilization advanced enough to hack into the genetics of a species on a distant planet--but who are it's authors and what new features or bug fixes does it provide?

Flu-like symptoms are probably just an unfortunate side effect. Things break during updates, it's an unfortunate fact of life. Maybe when a critical mass of people accept the update, they'll start building a giant transmitter for FRB's to let the sender know that the update was accepted.