Seriously. Code signing.
Viruses work because our cells contain machinery which will read, accept, and translate any RNA strands they see into proteins. Any RNA strands they see. Including RNA strands injected into our cells from viruses, or RNA strands transcribed from DNA injected into our cells from viruses.
Which is, from a security standpoint, pretty fracking stupid.
Code signing, I'm telling you. If our genetic material were signed with some sort of unique code that means "yes, this really does come from us, it's safe to translate this RNA and build this protein," and the transcribing and translating machinery would refuse to process RNA that wasn't signed, then viruses could inject their bits into our cells from now 'til Doomsday and it wouldn't mean diddly.
Code signing. Just one more reason why if we were designed by some Grand Creator, he wasn't very good at his job.