[Alex]

And alien viruses would be unlikely to know what to do with our biology to even cause a problem.

And the reason cross species infection is rare is not because of resistance (if that were true then we wouldn't catch diseases from other people either) but because diseases are usually evolved to very specific triggers that change across species. We have no protection against bird flu, it just generally doesn't do damage to us in its common form. It is only once it has mutated into a rare form that can infect both birds and humans that it becomes dangerous.