In the town of Ogden Marsh, Iowa, David Dutten (Timothy Olyphant), the local sheriff, is enjoying a baseball game when it is interrupted by a local resident, Rory Hamill, who enters the outfield with a shotgun. David, knowing Rory's history of alcoholism, attempts to dissuade him, but is forced to kill him when he raises his weapon. David and his wife Judy (Radha Mitchell), the community doctor, begin to notice other town residents exhibiting bizarre behavior, including listlessness and repetitive speech. The next night, a local farmer burns his house down with his wife and son trapped inside. When firefighters arrive, he is found mowing his lawn while his house burns.Learning of a pilot's body found in a swamp, David and his deputy Russell Clank (Joe Anderson) investigate it. They discover a military aircraft that crashed into the river a few days before, contaminating the drinking water. Suspecting a link to the town residents' odd behavior, David shuts off the town's water supply.Soon after, communication services are lost in town, and soldiers arrive to take all residents to quarantine at the high school. Everyone is examined for symptoms of infection. Judy does not pass the examination and is separated from David. She explains her fever is due to pregnancy, but is sedated anyway. David escapes evacuation and heads back to his office, encountering Russell. The two head for the school to free Judy. At the school, the infected townspeople breach the perimeter, and the military personnel evacuate, abandoning the civilians. Judy wakes up strapped to a gurney, as a crazed school director kills quarantined people one by one. David and Russell save her in time, and also find Becca (Danielle Panabaker), a hospital assistant.Unable to find a working vehicle, the four make their way out of town on foot. They encounter Becca's boyfriend, Scotty, at his farm. Soldiers raid the farm, shoot Scotty and his mother, and burn the bodies. They learn that the soldiers have been ordered to shoot all civilians who have potentially been exposed. The group repair an older patrol car in David's and Judy's garage, and are ambushed by the infected family of Rory. After a struggle, Russell furiously shoots the infected multiple times. This greatly disturbs Judy, who argues with David about Russell's state of mind. On the road, they are spotted by an attack helicopter and drive into a car wash for cover. The infected car wash workers attack and one drags Becca out of the car with a wrapped hose, breaking her neck. When the rest of the group leaves the car to help her, the helicopter destroys the car.Russell disables a passing government SUV with a police spike strip. The driver reveals the cargo plane contained "Trixie," a "Rhabdoviridae prototype" biological weapon. It was en route to Texas to be destroyed when the plane crashed. Enraged, Russell shoots the driver and threatens Judy and David. When confronted about his behavior, Russell realizes he is infected and, after being disarmed, begs to go on with Judy and David. He later dies while distracting soldiers at a roadblock, so that Judy and David can sneak past.David and Judy arrive at a truck stop to search for a vehicle, discovering that the military have also executed those who were evacuated. Fending off three infected, they escape in a semi-truck. Ogden Marsh is destroyed in a massive explosion as they flee, and their truck flips in the passing shockwave. As the couple walk towards Cedar Rapids, a view from a military satellite highlights first the couple, then the city, and the words "Initiate containment protocol" appear, signifying a new containment attempt.In the credits, Bruce Aune, a real newscaster from KCRG-TV 9 in Cedar Rapids, Iowa reports that an explosion originating from the Dakon Pendrill chemical plant started a massive fire in Ogden Marsh. He says a perimeter has been set and civilians are not being allowed into the area. A Trixie-infected individual appears on camera just before the signal is lost.
Output: Question: Who realizes that he is immune to the virus?, Answer: David