Viruses will attack anything they can successfully reproduce in, they have absolutely no preferences. They are binary, they survive and thrive or they don't. That's why some viruses can sucessfully jump species, and some can't, some viruses are able to survive in multiple hosts through dumb luck. Yes, they all need specific types of conditions in their hosts to survive, but they don't care if it's a big, a dog, a human, or an alien. If it gets what it needs, it'll go there.

Note: I'm not saying viruses don't have conditions they need met, they do, obviously. I'm saying they can attack an alien just as likely as they can a human.