Quote Originally Posted by Litmaster View Post
I think the answer is simple: Chuck just got through stabbing a bunch of Zeds with that spear, then used the same blood-soaked spear tip to stab Mark.

Boom, instant infection.
Well they did say at the end, "he's covered in blood" and then "most of its not even his blood. So I'm thinking its a Tommy situation where just a bit of blood can get into the wound and infect him.