There are a couple things that could lead to contamination. They have all had zombie juice on them at one time or another. The kid, Randal, was stuck in the barn where the walkers were kept. Probably plenty of zombie "stuff" in there and he was working the cuffs enough to create open wounds. Shame used the knife he stuck the zombie guard with to cut his own hand in the bus last week. Plenty of time to get the bug inside.
I think this is the end of the farm though. Hershel set the stage for it when he mentioned the swamp was drying up and the cattle herd was getting thinned out. The icing on it was Rick telling everyone to get the cars up to the house and all facing away from the house. No backing up, just get in start them and head out. I figure they will have another migration after the house gets overrun by the herd of walkers they ended the episode tonight with.
The only problem is the camp. Randal said they were about 5 miles away on the highway so there is the possibility that they could run into the other group.
I figure out that from now on they have to shoot everyone that dies in the head. You might not turn into a zombie until you die but it sure looks like you do go zombie.
