For a good mindscrew, how's about it's possible for someone to be a carrier for the disease, but not affected by it themselves? Like, this guy might be safe himself, but anyone he kisses or bleeds on is screwed unless they, too, are carriers? 28 Weeks Later had a pretty nifty idea there.
Out of curiousity, what's the incubation period on this disease? You might've said it, but I missed it.