I always felt that it was the fact that the undead are not really dangerous in small groups. They are easy to escape from, but it only takes a little nip-some broken skin and saliva, and that guy who narrowly escaped is now a ticking time bomb heading back to base. So let's say a group of 10 people head through the city; One gets bit in the leg by a lurking zombie, which is killed by someone else. As they are helping him someone else is jumped from behind, getting a small scratch but managing to push it down. Getting away and running towards a shopping center, a small herd is at the door. pushing through it winds up with a few more minor injuries, but the group manages to get inside and barricade the doors. Someone stays up to watch, while the others sleep. Sometime in the night, the first guy bit dies and reanimates, lurching out of the darkness and surprising the guard, tearing at his neck. The noise wakes the party, and they attack the zombie drawing it away from it's now infected and quickly dying prey. Even if the group kill this first zombie, they will need to also kill the guard on duty and (assuming someone has figured it out already) others wounded during the day lest they also shortly turn and become threats. Except for the young pretty girl that is being protected by her ignorant boyfriend ("nah man, she just cut her arm on a fence") who winds up turning the next day and biting the boyfriend.... Cycle repeats. At this point, a 10 person party could be reduced to 3 within a day and only a few actual kills under their belts. In the beginning, people were not exactly aware that any little injury from these jerks will spread like "Wildfire", therefore too many people sustained injury in the early days and were likely given peroxide or antibiotics... Neither of which would help at all. The end result was a population that probably fought hard early on, then realized they should have been fighting smart.

So there are a lot of scenarios where people are entirely consumed by the dead, as the horse was. But remember, Rick was there and made it away while they ate his steed... Had they even caused a minor wound on Rick as he was pushing through them he would have wound up just like the tank driver, or even not gotten sick until they got him to the quarry, where he would likely turn in his sleep and at the very least wound Lori or Carl--- Cycle repeats. To make matters worse, even if it is just little ol' granny having a stroke from all the stress, you got another problem on your hands...

 

I've watched season 1 a few times, and I can't recall a walker/zombie taking a bite out of a human and saying "whew...I'm full" or acting like they didn't like the taste. They keep going for more. You'll also see people who get bit but managed to get out without dying right away (Jim). The characters in season 1 pretty much knew the basic rules of infection, but during the initial outbreak I'm sure most people had no clue thereby causing numerous situations in which people turned.

You'll get some more explanation/understanding of the basic rules and scenarios by which people turn from Season 2. It would be difficult to answer any further without spoiling (and I wont).

 

Yes, this happens mostly to those self-conscious walkers though. Y'know, the ones who really go all out on their looks and judge the others who dare to be over 100lbs / 45kg.

 

I would imagine if the brain is left in tact after a person is devoured, they would turn. The walkers aren't creating an army of themselves either because they are not consciously aware of what's going on around them. They are only driven by the instinct to feed and that's it. Which is also why they might seem like an army when they're really only going after the same target. They detect something live and that's it. Doesn't matter what it is, it's food to them.

 

As long as the brain is left intact, the person will turn.

that being said, Darabont's rules about walkers are different from walkers in later seasons. His are more intelligent and dangerous. Later on it makes more sense that a person could get bit and then escape, to become a time bomb within their group that they didn't tell they were bitten.



Also, I won't say what... But Jenner told Rick another reason why there are so many walkers. You don't get to learn it until the end of season 2.

 

That's right! Along with the outbreak causing corpses to rise, all of the species become female and huddle together in herds for mating season which is usually between Summer and Autumn. Hence why we see so many of them on the show and how well everything seems to go inbetween seasons. The way they give birth is actually by having the children claw and tear their way out of the womb and front of the torso, for my own personal image i like to think of it as similar to the Alien's from the Ridley Scott series.

 

Many people would get bitten but then escape, unless they were surrounded by multiple walkers, as others have stated. Children would be less likely to get away, even from a single walker, and would be devoured, which is a neat way to explain the distinct lack of child walkers. But even if 50% of people got devoured, there would still be over 150 million walkers in the U.S. alone.