Back to "everyone is infected". Again, at the start of the season, dead bodies on the road and in cars that weren't walkers. (Carl finds some knives in one car with a corpse that IIRC, didn't have a head wound.)
This has been bugging me ever since the word got out that it was very possible they were all infected. The only explanation I can fathom that would make sense is that at some point a walker that doesn't get any food is going to die (for real permanent) and/or the ones trapped in the cars were basically mummified to a point past reanimation.