Hmm, that's a fair point. OTOH, if your virus were intended to target one or more GPL'd programs specifically (if, for example, MS decided to release a virus to go after Cygnus), then it could be considered an attempt to distribute a derivative work, just as the original NeXT Objective-C compiler was. NeXT carefully tried to separate their front end from the rest of GCC, and make the users link it in manually, but after their lawyers talked to the FSF lawyers, they quickly backed down and released their formerly binary-only front end under the GPL. Which is why GCC includes an Objective-C compiler today. On paper, it may look like NeXT had found a perfect way around the GPL, but the law takes intent into account, and it was clearly their intent to distribute a derivative work, even though technically all they distributed was their own work and a set of instructions.
Note that this could be a problem for anyone writing a virus to target GNU/Linux, at least if it were at all dependent on either the kernel or core GNU facilities. A POSIX-compliant or Single-Unix-Spec-compliant virus that just happened to be able to target Linux, though, would probably be ok. But since viruses often take advantage of flaws in an implementation, it would probably be a lot harder to create a successful virus that ignored implementation completely.
Heck, in the end, it's probably just easier and safer to go ahead and GPL your virus. There's no downside, and it may have unexpected benefits. :)