In an interview over at Vulture, Robert Kirkman talks about what this mysterious virus is:
Pulling in a new threat in your fourth season, I think, is a good thing. Also, it’s important to know that this is really just some form of the Spanish Flu. It’s something that exists in our everyday world now; it’s a time where there is no modern medicine. Scott Gimple, the new showrunner, really wanted to explore that new possibility of the simple things, the really little things, coming to devastate these people. That’s what we’re going to explore.
He also went on to say that this virus will infect and eliminate many of the new cast members we met in the premiere.
And, brace yourselves, Greg Nicotero said something that might greatly concern Team Daryl:
At the end, when Patrick dies, you realize that this is not an isolated incident. [Rick’s] pig is dead, we see the walker at the fence … it’s a ticking time bomb. Now, all of a sudden, [the virus is] right in the middle of the prison. We set up some great beats. Norman [Reedus, as Daryl] set up a great beat when he licks his fingers then shakes Patrick’s hand. You look at it and go, ‘Wait a minute, Daryl licked his fingers and shook Patrick’s hand and now Patrick is [dead].’
We’ll just have to wait and see! Of course, no one is safe in this horrible situation, including those little kids that Carol is hell-bent on teaching to defend themselves. Nicotero ended the interview saying that they’re up to episode thirteen and “People better hold on.”