One more thing:
I strongly urge you to look into Spanish Conquistador's virus. I kid you not, that thing is a monster. A monster in its ability to leave SC unscathed. It's ability to kill off other species (even species with intelligent anti-viral genes). It's ability to create a self propogating virus that quickly takes over. It's masterful (if I do say so myself) use of the stack. It's definately the most deadly virus to date (deadly not because it causes instant kills (tried that, it wasn't as successful) but because it manages to leave the victim going for a few hundred cycles, reproducing like mad and creating new viral particles in each daughter cell).