[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?