All Out War
In the defense of the Hilltop, Marcus is stabbed by an unnamed Savior. He is last seen being brought to the Hilltop mansion by Michonne to get checked up on by Harlan.
Later, as more and more injured begin to have the symptoms as if they were bitten, the survivors discover that The Saviors have mucked up their weapons using zombie guts. Harlan quickly comes to the realization that anyone who is injured, no matter how minor, will die. Many of the injured are put down or pass soon after the discovery, it is unknown what happened to Marcus, but it is presumed he succumbed to his injury or was put down.