Talk to the Sheriff and tell him YES to start the mission. Head to the first Infestation and enter the house then kill the required number of Screamers first, then go outside, ride a car and just keep running over the zombies around the perimeter of the marked house until the Infestation is cleared.
A second Infestation will appear. However almost simultaneously a lady will call you asking for help. Ignore the next Infestation and just head over the marked houses on the map. Search all of them until you find the damsel in distress.