[burnte]

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.

[mrkstu]

Viruses have evolved to hijack DNA- I'd find it highly unlikely a truly alien biology would be susceptible to earth viruses. Maybe some truly primitive precursor virus may have been able to, but I'd think any of those would be long gone.

[burnte]

We have no idea if they'll be DNA based or not, but there's a nonzero chance they might. If Earth found DNA the easiest path, so might another world.

[TheAdamAndChe]

DNA is "translated" from code to proteins using codons. There's a very good chance that the codons used would be completely different from our own, disrupting viral replication.

[WalterBright]

But will the alien viruses attack us?

[charwalker]

If they operate on a chemical/biological structure that humans have then yes. Just nobody go designing viruses for hypothetical aliens, ok?

[maxerickson]

We don't really know whether other forms of life would use proteins, or make the same ones the same way we do.

Both of those things are important to all of the viruses we have experience with.

[burnte]

Proteins are just groups of amino acids which occur in stellar nebulae. It could easily be a widely used building block of life.

[maxerickson]

I'm saying they can attack an alien just as likely as they can a human.

For that to be true, you need some governing rule of life, not 'could easily be a widely used'.

(You'll note, I phrased my other reply in terms of us not knowing)

[kiba]

Yes, viruses don't care. But at the same time, I am doubtful that they could.