The Tank is a massively mutated Special Infected from Left 4 Dead. Through unknown circumstances, the Green Flu has cause certain male Infected to develop hypertrophy, expanding their muscle mass in their upper bodies. The overdevelopment of their muscle tissue has caused the subjects' upper bodies to grow to ridiculous proportions, producing a sizeable hump and the deformation of the arms. The musculature of the chest has swollen so much that it has even enveloped the subject's lower jaw, making it appear that the lower jaw has been severed.
Despite their size, Tanks are fast and agile, though the growth of their arms forces them to drag their knuckles along much like gorillas do. Their immense strength allows them to smash through obstacles with ease and pick up large, heavy objects and throw them. The hardened bone and muscle throughout the body is also highly resistant to gunfire; even the skull has been hardened by the mutation and shots to the head are no more effective than anywhere else. It takes a staggering amount of firepower to kill a Tank and fighting one alone is utterly suicidal.