She participated in the raid against the Umbrella Facility in Caliban Cove. She was infected with the T-virus after touching a spot of dried blood on a gurney and then scratching her eye and subsequently turns into a "zombie" (Tri-squad). Unlike other zombies that appear in the story, these type of zombies carry some degree of cognitive skills, as they are able to take orders, fire weapons and even pose the ability to speak. However, Karen eventually becomes a regular zombie.
She dies at Caliban Cove when she gets disposed of by Dr. Nicolas Griffith in order to be replaced by a more fitting, stronger and robuster "specimen", Steve Lopez. She carried an old Hand Grenade for luck, which is used by Rebecca Chambers and Captain David Trapp when they become trapped, along with John Andrews.