The Infected Chubbies are special Infected.
"His infected chubby body can support a huge amount of damage before he dies."
An Infected Chubby can be a somewhat difficult enemy early in the game due to its high hitpoints, some armor, and the speed of a runner infected.
They can spawn from Doom Villages and from later waves. They usually spawn on the outer edges of the map and are a force to be reckoned with if you are not prepared. They can also be a part of swarm.
Chubby also make a problem in late-game, as their high HP makes them higher priority targets for higher-level targeting than Harpies and Venoms. High single-target damage, like lots of sniper towers, are recommended.