Walking Dead Season 2

[James In NC]

If you back up a couple of episodes, Shane was nursing an injury on his arm, one that looked like a bite. I remember the scene where he was sitting at the water pump cleaning it and the missing kids mother came walking up. This would have been before they opened the barn.

So maybe he has been infected for some time and the virus had enough of a hold on him that it reanimated his very quickly.