I don't find it far-fetched to have multiple survivors in the same group who might have a special rare immunity, at least at this stage in the story. If there's some kind of genetic immunity out there that prevents turning, it becomes a Darwinian advantage, allowing them to survive at better rates than the "normals." I'm thinking of the premise of survival in Stephen King's The Stand
I wouldn't be surprised to find a few more examples like Saul and Tanya and maybe even Datu, those who have already been exposed, but hid their exposure for fear of being run off, whose "immunity" is the reason they are still alive.
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