I'm not sure if anyone has been talking about this here. Speculation/Spoilers below...
From a recent interview with David Morrissey we find out a little more info on this new threat for Season 4 that they have been hinting at for a while now!
From the interview:
How would you describe the overall arc of season four, where are we headed?
Well we don't know where we're heading. We get the scripts on a weekly basis. So we don't know where we're going. This new threat is the thing that overall is the thing for us. The zombies are the same threat that thy've always been but with this added extra that makes the world a very, very dangerous place. And people start to sort of react in different ways.
You say "threat," not "person" or "character." Why is that?
It's not a threat you're going to be able to kill very easily. So it's something that happens within the world. Another thing. And that is very dangerous. It's very dangerous and I think when you see it you'll… there's a lot of identity with it, for us in our world. You'll get it when you see it. Each person reacts to it very differently.
In the trailer we seen that Carol, Hershel and Daryl were wearing a bandana over their face at times. We also saw some strange new walkers, ones with bleeding eye sockets. Could this be some kind of viral epidemic or something similar?!
This could possibily mean that there isn't a saboteur in the prison and this is how walkers got inside; dying from some virus and turning.
These guys just can't catch a break...
So what do you guys think? What could it possibly be?!?