The games are overseen by an ' AI Director', designed to give the players a more dramatic experience based on their performance, penalizing players for stalling while rewarding players with special weapons by taking longer or riskier paths. Certain unique infected creatures pose a more difficult challenge, typically requiring teamwork to take down effectively. Players use a combination of melee weapons, firearms, and thrown objects to fend off attacks from the bulk of the infected creatures, while using an assortment of healing items to keep their group alive.
The games encourage cooperative play between up to four players, each taking the role of one of the survivor characters and the computer controlling any unassigned characters. Set in the days after a pandemic outbreak of a viral strain transforming people into zombie-like feral creatures, the games follow the adventures of four survivors attempting to reach safe houses and military rescue while fending off the attacking hordes. Left 4 Dead is a series of cooperative first-person shooter survival horror video games published by Valve.
