Plot
Henry, a hermit walking in the woods, encounters his dog, dead from a blood infection, and becomes infected himself from contact with his dog's blood.
College students Jeff, Marcy, Paul, Karen, and Bert take a vacation to a remote cabin to celebrate October break. Bert leaves to hunt squirrels but accidentally shoots the now disfigured and bloody Henry. Despite Henry's pleas for aid, Bert flees and remains silent about the incident.
As the friends gather around a campfire that night, they are joined by a friendly drifter named Grimm and his pet dog, Dr. Mambo. When it rains, Grimm leaves with his dog. While the friends wait for Grimm, Henry stumbles into their camp begging for help. When Bert shuts the door on the sick hermit, he tries to steal the group's car while vomiting blood. When Henry approaches Marcy and Karen, Paul accidentally sets him on fire.
The next day, while seeking help fixing their truck, Jeff and Bert encounter a butcher, but leave after learning that she is Henry's cousin. Paul calls police Deputy Winston, who promises to send up a tow truck. Paul tries comforting Karen, who is upset over Henry's death. After calming her down, Paul attempts to have sex with her, only to discover a bloody, infected wound on her thigh. The group isolates her in a shed.
After fixing the truck, Bert coughs up blood but hides it from the others and drives off after Paul and Jeff find out. Jeff takes the remaining beer and leaves, terrified of becoming infected. Bert seeks help at a convenience store but angers the owner after his disturbed son, Dennis, bites him. Bert flees, chased by Dennis's father and two friends. At the cabin, Marcy worries that they will all contract the disease. Paul comforts her and they impulsively have sex. Soon after he leaves the cabin to find Jeff, while Marcy takes a bath, crying. As she shaves her legs, the flesh begins to peel off and she runs outside in a panic, where she is mauled and eaten alive by Dr. Mambo.
Paul discovers Henry's body floating in the reservoir, and realizes that the infection is spreading through the water supply. Returning to the cabin, Paul finds Marcy's bloody remains and sees Dr. Mambo feeding on a weakened Karen, whose face has now become fully decayed by the infection. After killing the dog with Bert's gun, Paul bludgeons Karen to death with a shovel in a mercy killing.
A dying Bert returns to the cabin pursued by Dennis's father and his two companions. The posse kills Bert, and Paul kills all three of them. Paul looks for Jeff and instead finds Grimm's corpse in a cave. Paul takes the convenience store's truck, and, while driving, discovers that he is infected before hitting a deer. He reunites with Deputy Winston, who is partying with underage drinkers. Paul requests a ride to the hospital, but before the group departs, Winston is ordered to kill on sight several infected people on a killing spree.
Paul attacks but does not kill Winston. As he attempts to hitch a ride, he falls unconscious and gets picked up by a passing vehicle, whose occupants drop him off at a hospital. The doctors request Deputy Winston to transfer Paul to another medical facility.
The next day, Jeff, who has been hiding out and drinking in the woods, returns to the cabin. Initially crying after seeing the remains of his friends, he becomes ecstatic upon realizing that he survived. As he raises his arms in victory, he is killed by local police and his body burned with the remains of Bert and Karen. The sheriff asks Winston if he took care of Paul.
Two kids get water from the lake for their lemonade stand, not realizing that a barely alive Paul is in the lake and contaminating the water. The sheriff and his deputies stop by the convenience store, owned by the children's grandfather, where they buy glasses of lemonade.
