Well Shane's gone.
Sad he went out that way, but I'm thinking his crazed attitude wasn't really his own. It's just a theory, but I'm wondering if he'd been infected for awhile (even before the blood/ saliva bus incident.) Remember when they all ended up at that CDC complex? They guy who was running it, if memory serves me right, took blood test of all of them. Now, when they were running out before he blew that shit up he whispered something to Rick. I'm thinking he whispered something along the lines of Shane was infected, or told him how the virus spread.
I have a few theories on how it spreads/starts:
#1 It spreads via blood and saliva to open wounds. Shane contracted on the bus or in an earlier event.
#2 Anyone who comes into contact with a zombie becomes a carrier. Or the infection occured via insects or something airborne. When they die they turn. Name one character who has been killed with a brain injury and hasn't come back.
But I'm thinking this as the most likely.
#3 Inherently shitty people come back as people eaters when they die.