1-Dale had to die or wander off because when the shows director/producer/head dude/whatever left after the first season Dale didn't like it and asked to leave. Apparently things got better (can they really be that bad when you get to work on a set of a freakin zombie apocalypse?!?) and he wanted to stay but they had already ended his contract and he had to become zombie carnage.
2-Shane is reportedly dying at the end of this season. Now, all the spoilers could be a red herring and he may just roam off to hang out by himself long-term, but either way he is going to do a new show. The leading spoiler for Shane is that Rick kills him as they struggle (and Im guessing Lori has something to do with it ). I know a lot of folks hate Shane but I think he is an incredibly dynamic, interesting character played by a phenomenal actor. Further, I feel like they have purposely made Shane a bit over-the-top crazy this season to justify Rick's murder when it comes along. This is a huge shame and can only be redeemed if Daryl gets more screen-time.
3- In the comics Lori bites the dust--PLEASE LET THAT HAPPEN SOON!
4-There is a good chance that the doctor at the CDC told Rick that they were all infected with the virus that causes zombieism as Rick and co. were escaping. In the comics everyone is infected with the virus and if you die you turn into a walker (unless you die from a massive brain injury) even if you haven't had any contact with a zombie. Also, in the Talking Dead they revealed that the title "The Walking Dead" actually refers to the living folks--not the walkers. Does that make your brain go WHOA! or what

...how's that for awesome symbolism and thought-provoking subject matter?