Green Flu is a fictional disease featured in the video game series Left 4 Dead. It is characterized by rapid infection and transformation of individuals into aggressive, zombie-like creatures known as Infected. The disease spreads through bites and contact with infected bodily fluids. The origins of Green Flu are not fully explained in the game, but it is suggested to be a viral outbreak that affects the brain, leading to violent behavior. Survivors must navigate through dangerous environments while avoiding or combating the Infected to stay alive.