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Keep in mind that Rick's group got to the farm maybe a month and a half after the outbreak started - maybe less - and left after a few weeks (Lori wasn't visibly pregnant when they left).

THE TIMING:

Keep in mind that Rick's group got to the farm maybe a month and a half after the outbreak started - maybe less - and left after only a few weeks (Lori wasn't visibly pregnant when they left).

THE HERDS:

ZOMBIE INTELLIGENCE:

Part of the subtext of your question relates to the intelligence of zombies; you seem to be dangerously close to overestimating what the zombie brain is capable of. They are stupider than the stupidest person who ever lived. They don't think - they are single-minded (if it even makes sense to talk about zombies having "minds" of any kind), and they can only sense prey, chase it, grab it, bite it, and eat it.

This is what happened with the herd that attacked the farm. A few zombies saw a helicopter and started chasing it. Being a helicopter, it vanished out of sight very quickly, of course, but zombies are REALLY stupid, so they kept walking in the direction they last saw the helicopter going. Other zombies saw this group walking, assumed that they were heading towards food, and joined the group. They all forgot what they were chasing, but saw the rest of their herd moving and assumed that the other zombies knew there was food ahead, and this kept all of them moving in the same direction for days. Then, at exactly the right - or wrong - time, Carl fired his gun. The herd heard the shot and changed course - they turned and went towards the sound of the gunshot. If Carl hadn't fired the gun, they would have walked right past the farm. As I said, the herd attacking the farm was pure (bad) luck. And the fact that no herds had attacked it before that was pure (good) luck.

In the first two episodes of season 1 we saw zombies do three things that infuriated zombie fanatics like myself around the world - Morgan's zombified wife turning a door knob, a zombified child picking up a teddy bear, and one of the zombies in the herd outside the department store using a rock to break a glass door. These things would NEVER happen in the comics, because Kirkman is a zombie purist. I was also annoyed by the walker sitting down on the bus (unless that is where he died, he would have gotten off the bus and/or stood up, and since zombies don't care about comfort, if he sat down again it would have been on the floor - they don't know what chairs are), and by the zombie trying to climb the fire escape ladder (they can't climb and don't know what ladders are), but at least the ladder zombie failed before he got off the ground. We zombie freaks take these things very seriously. I'll stop now, lest I turn this into a rant.

This is what happened with the herd that attacked the farm. A few zombies saw a helicopter and started chasing it. Being a helicopter, it was moving so fast that it vanished out of sight very quickly, of course, but zombies are REALLY stupid, so they kept walking in the direction they last saw the helicopter going. Other zombies saw this group walking, assumed that they were heading towards food, and joined the group. They all forgot what they were chasing, but each zombie saw the rest of its herd moving and assumed that the other zombies knew there was food ahead, and this kept all of them moving in the same direction for days. Then, at exactly the right - or wrong - time, Carl fired his gun. The herd heard the shot and changed course - they turned and went towards the sound of the gunshot. If Carl hadn't fired the gun, they would have walked right past the farm. As I said, the herd attacking the farm was pure (bad) luck. And the fact that no herds had attacked it before that was pure (good) luck.

Note: In the first two episodes of season 1 we saw zombies do three things that infuriated zombie fanatics like myself around the world - Morgan's zombified wife turning a door knob, a zombified child picking up a teddy bear, and one of the zombies in the herd outside the department store using a rock to break a glass door. These things would NEVER happen in the comics, because Kirkman is a zombie purist. I was also annoyed by the walker sitting down on the bus (unless that is where he died, he would have gotten off the bus and/or stood up, and since zombies don't care about comfort, if he sat down again it would have been on the floor - they don't know what chairs are), and by the zombie trying to climb the fire escape ladder (they can't climb and don't know what ladders are), but at least the ladder zombie failed before he got off the ground. We zombie freaks take these things very seriously. I'll stop now, lest I turn this into a rant.

In fact, zombies are so single-minded that even when they are in the middle of eating someone they killed 5 minutes earlier, if another person walks by, they will abandon their meal and chase after the new prey. Since they are dead, they can't digest what they ear, and eating doesn't benefit them in any way. A zombie is basically a vehicle the virus uses to infect new hosts. It makes the zombie feel constant hunger, which makes the zombie bite people, which spreads the virus to the bitten person, and so on. So it is in the virus' best interests to make the zombie host bite people, but NOT let them eat the entire body. The virus only benefits when the bitten person is left more or less intact, so they can turn and begin biting people as well, thereby spreading the virus to new hosts. When a zombie bites someone, and the bitten person is completely eaten, the virus doesn't benefit at all.