Infection and the dead rising can paralyze you with fear, but fear is a luxury after my most recent findings. We now know infected humans and the risen dead evolve... and fast. From what I could observe they evolve through cannibalistic means. As a biological analyst I have never observed anything like this on our previously unfrozen world.
Early on we realized that a Hype, an infected human driven mad and overly violent, killed with a fully intact brain will rise again after death. These undead beings, named Risens, are more docile than their human counterparts. Hypes and Risens will only attack each other to evolve their species but otherwise mostly do not interact.
I have tracked their evolutionary patterns to the best of my abilities. From what I gather the evolutionary tree splits directly at the base leading to two main and more specific lines of evolution. First is what I call the Living Line starting with the Hypes and following an evolutionary path of weaker infection spreading creatures. The second is the Reanimated Line starting with Risens and expounding on strength and more recently radiation mutations.
I can only guess at where these leaps in evolution will go, but from what I have observed there is no end in sight.