Was Mark infected before beeing killed by Selena?
Even if he had a wound on his arm, we don’t see him getting bitten, and even half a minute after the attack, just before angry chick kills him with a machete, he was showing no symptoms of an infection.
When others got infected it took no more than 5 seconds to turn them into raging "zombies".
So yeah, my guess would be, he cut himself either with his knife or with that glass laying around and then she killed him even though he wasn’t infected.