A demonic invasion has occurred at one of the UAC's hazardous waste testing and disposal facilities. Reports gathered from intermittent transmissions have indicated that the hazardous materials located within the facility have infected some of the creatures and caused them to mutate. Enter the underground sewer network, push through the ruins of the abandoned town, and infiltrate the facility itself. Stop the invasion, and stop the spread of the infection.
One of the new monsters, the Infected Trooper. They take more damage than your average zombie, and dish more out, but they are also very slow.