*Incoming wall of text*
I've been looking the Lee bite scene over a bit, and I think its a scratch, not a bite. First of all its was very quick, and when he 'bit' Lee his head went straight down. It's not possible to sink in deep enough and pull out downwards without taking a piece of flesh with you.
Second, If you look closely, it doesn't quite look like a bite. Take for example the bottom of the 'bite'. There is no site of the mandible (Lower teeth). When something bites, the marks are the maxilla and the mandible. The bottom doesn't show lower mark of teeth, only 2 corners that's not teeth shape.
Thirdly, the marks on the top is vertical, much scratch like.
Fourthly, The top are 4 marks, which are the primary, middle, ring and pinky finger. On the side there is another mark, which is of the thumb. Most of the 'bite' is in a mouth like shape except that part. A mouth can't reach that far and even if a zombies mouth can there is not teeth there (And I'm pretty sure zombies don't grow extra teeth). Also if you look closely the zombie lunges at Lee with his right hand first. In slow-motion, you can see the blood splat before the zombies mouth reaches Lee, and instead the hand. And the right hand's thumb is on the left, the same side of the mark on Lee's 'bite'.
Lastly, if you get bitten, I'm pretty sure you will fell the pain and immediantly look at the mark. Lee noticed he got 'bit' when his blood landed on Clem's hat.
My theory: The zombie scratched him and it was merely skin deep. Because in his shock and fear he didn't notice the mark until he saw the blood. It is known that a person in shock, fear or extreme stress are less likely to feel mild pain. A skin deep scratch isn't very painful so in Lee's situations (Him being in shock of the zombie attack and his worry of Clem) it was very unlikely that he felt such minor pain. If it was a bite, he would obviously feel it no matter how much shock. I've been bitten by a dog and my annoying little stepbrother. Both were skin deep and it hurt like hell. When I was helping in the garden I got scratched while working with thorn bushes. I first noticed the bleeding dripped over my shoes. So that's my theory. I'll be honest, I've been very pessimistic, and a bit annoyed at people when they are like 'Telltale can surprise us, Lee may live!' but now I think Lee might actually survive. Unless you can get infected by a zombie scratch. I know it is like that in other zombie games and films, but it was never stated in TWD universe that a scratch can turn you, a bite yes, but not a scratch. *End Wall of Text*
TL;DR: Lee was scratched, not bitten.
This is just a ray of hope, that I hope is true.