>Anything from another planet wouldn’t have developed in a way that lets it get past our immune system not because or complexity, but because of lacking the shared evolutionary history.

Based on? Rabbits didn't co-evolve with australia's deadly fauna, and yet...

The rabbit example is a matter of ecology. A viral infection is a matter of immunology.

They are not analogous and depend on completely different factors.

If you have ever done any work or reading on immunology then you wouldn't think it to be far fetched to claim that immunology IS in fact ecology.

Indeed, microecology is a thing.

They did if you go back far enough.

We are talking compatibility on a protein level.

Well, this also works from the opposite perspective: we have evolved to combat earth virusses, but haven't evolved to combat anything from another planet. Lacking the shared evolutionary history may leave us more vulnerable.

An alien virus would have to be very lucky indeed to have the necessary machinery to dock to our cell's and inject itself into the cells replication mechanism. Viruses are often not even able to bridge the gap between relatively closely related species. A alien virus being able to infect Earth life seems vastly less likely than a virus that infects sea sponges being able to infect humans and nobody really worries about that.

It wouldn't have to be some form of virus to be deadly though, right? Something like prions or unlike anything we know could be equally (or more) deadly to us or even all life on earth. Our imagination is quite limited in this domain and well let's hope it stays this way..

(not that I find any such scenario to be even remotely likely)

Edit: well, that's basically what's being said in a parent comment

Yes, alien bacteria have a much better chance at surviving.

if an alien life-form is DNA based, maybe other similarities are shared with Earth based-lifeforms, even if it doesn't share an evolutionary past with it.