Left 4 Dead Intro

The following event begins in Pennsylvania, following an outbreak of the so-called Green Flu, a highly contagious virus whose symptoms include significantly increased aggression and the loss of many higher brain functions within its victim.
Two weeks after the start of the epidemic, four immune individuals; Bill, Zoey, Francis and Louis are in Fairfield, where Bill discovers the remains of an unusual Infected, signifying that they are evolving.