In The Last of Us a fungal infection has devastated human beings. The pandemic corrupts the mind and has left most of the population grotesquely disfigured and relentlessly aggressive. This chapter focuses on artificial intelligence (AI) behind the Infected. The Infected are humans who have succumbed to the parasitic Cordyceps fungus. The AI for a specific type of character is implemented as a set of skills that invoke a hierarchy of behaviors. Both skills and behaviors are implemented as straightforward finite-state machines. Skills and behaviors are a small part of a complex AI character, but they provide the decision making needed to make the character respond convincingly to the player. Using tuning values that were independent of the type of character allowed all of the Infected to share the same skills and behaviors but respond in ways that were unique and interesting. The wander skill is the lowest priority skill for all of the Infected character types.