[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)