Calculated

From my understanding from the CDC episode in season 1 the new zombie organ created in the brain continues to send signals to organs for them to pseudo function via the nervous system. So they don't decay as fast as you would think if you left a body out in the sun for 6 months. But you start to see those anorexic zombies who haven't been gorging on people and appear very frail. But I am probably way off on my speculation.

Well keep in mind the bites/scratches from zombies kill you via blood loss, weak immune system, and blood poisoning. They(humans) all carry virus, so it's not like they need to be infected.