I feel like there are two schools of thought on a zombie bite, the first one being that if you get bit, you're dead and nothing can save you, even amputation. The second one is if you get bit on an extremity (hands, arm, legs, etc.) if you can cut it off fast enough, the infection won't spread and you will live. We all know Hershel got bitten and they cut his leg off to save him from succumbing to the infection so clearly TWD subscribes to the second theory.

So here's what's bothering me, Merle shot Michonne in the leg, causing an open wound. Later in the fight she stabs a walker and gets the guts and gore and stuff all over her, including (I'm assuming) on her leg. Why wouldn't that count as a "bite", I would think that an open wound getting all that blood and gore in it would warrant a transfer of the infection like with a bite.

TL,DR Michonne gets shot, gets zombie blood on her, why doesn't she get infected?

Edit So what I'm getting from most of these posts is that just simply getting bitten won't cause you to turn, but rather the infection/blood loss that results from being bitten causes death which ultimately causes you to turn. Also the bacteria in a zombies mouth is probably a lot higher due to lack of hygiene and all the various raw meat they consume which causes the bite to be much more toxic than a typical bite from say another human or animal. Combine this very strong infection from the bite with a lack of adequate medical care (in most cases) and the victim will fall ill, eventually dieing and then re-animating as a walker. These are some great theories, keep 'em coming!