"Those after the great bombing, who have given up on themselves, or waste their life on nothing but negativity, eventually succumb to the negativity virus. They will only feel that emotion, and try to even spread the disease. There is no cure to this, as of yet."

From left to right:
Plague
Smoker
Stalker
Shell

They feed off of sludge and corpses.
Plagues are hunters, Smokers Poison the air around them, Stalkers follow people in the shadows, and Shells often lie around, and are really slow.