Quote Originally Posted by PhysicsLB View Post
What if each different zombie is a different strain of the original infection?

The vector is evolving with each infection, which creates these offshoot species. This would explain why jumpers disappeared: they have a higher caloric cost to function, but their ability to find prey dwindled as the population of humans dropped, leading to their "extinction". This could also explain why we weren't seeing anything like behemoths very early on, and why runners and jumpers (essentially souped up biters) were the first specials to show up. Now we have the "Little Ones", the cutting-edge in biological manipulation. They hold all the best traits of all the previous species: speed, agility, toughness, and intelligence! And they self propagate! The only thing any crazed evil genius would ever need!

In all seriousness though, it appears the "virus" is trying to improve the host body and, by extension, it's propagation through accelerated evolution. A "trial and error" of different assets. Runners and jumpers showed that imrpoved athleticism was effective in the early stages of a crumbled society. Behemoths showed that better toughness and strength made it easier to bust through defenses, and survive prolonged exposure to a settlement. The smart ones have obvious advantages, and the Little Ones combine all these aspects.
In short: Darwin was right.