Cabin Fever

A hermit walking in the woods comes across his dog. He tries to get the dog's attention, but the dog is dead due to a bloody infection, and the hermit comes into contact with the infected blood.
A group of college students, Jeff, Marcy, Paul, Karen, and Burt, take a vacation to a remote cabin in the woods to celebrate the start of spring break. Jeff and Marcy are in a physical relationship, while Paul is trying to get his childhood friend Karen to go out with him. Burt is the proverbial "fifth wheel" of the group. Upon their visit to a local convenience store, they meet an unusual boy named Dennis, who has a tendency to bite people. When the five get to the cabin, Jeff and Marcy have sex while Paul and Karen go for a swim. Meanwhile, Burt goes out to shoot squirrels with his rifle, but ends up shooting the hermit, now disfigured and bloody. The hermit flees, but Burt does not tell anyone about the incident.
That night, the friends gather around a campfire, where they are joined by a friendly drifter named Justin, who prefers to be called Grimm, and his angry dog, Dr. Mambo. After they share some marijuana, it starts to rain, so Grimm leaves with his dog to pack up his stuff. While the friends wait for Grimm indoors, the hermit returns, in a much worse state than before and begging for help. Burt shuts the door on the sick hermit, who then tries to steal the group's car, while vomiting blood all over it. When the hermit threatens Marcy and Karen, Paul accidentally sets the hermit on fire while trying to ward him off. The group looks for help the next day. Jeff and Burt find a helpful neighbor, but leave when they find out she's the dead hermit's cousin. To make matters worse, Dr. Mambo begins harassing the group. Paul later gets assistance from police Deputy Winston, who promises to send up a tow truck, but in the meantime encourages Paul to have a good time and party. Paul goes on to comfort Karen, who is upset over the killing of the hermit. After he calms her down, it appears Paul is finally going to have sex with her, but when he goes to touch her, he discovers a massive bloody infection on her thigh. The group isolates her in a shed outside of the cabin.
After fixing the truck, Burt begins coughing up blood, revealing he is infected, but does not tell the others. After Karen vomits blood on the truck, Jeff takes the group's remaining beer and leaves. Burt goes back to the convenience store to get help, but incurs the wrath of Dennis's father after inadvertently infecting Dennis with the disease when Dennis bites him. Showing more signs of infection, Burt flees, chased by Dennis's father and two friends. After moving Karen back to the shed, Marcy and Paul return to the cabin and have unprotected sex on a whim. Immediately regretting the affair, Paul leaves while Marcy, who developed infected sores on her back during sex, takes a bath, crying. When she shaves her legs the flesh begins to peel off and she runs outside in a panic, where she is chased and killed by Dr. Mambo who tears her apart.
Meanwhile, Paul discovers the dead body of the hermit floating in a reservoir, revealing the infection has been spreading through the drinking water. After racing back to the cabin, Paul finds Marcy's gory remains and Dr. Mambo feeding on Karen in the shed. After killing Dr. Mambo with Burt's gun, Paul puts Karen out of her misery by beating her to death with a shovel. A dying Burt later returns to the cabin, still pursued by Dennis's father and his two companions. Burt is brutally killed by the posse who shoot him, before Paul kills all three of them. After killing Dennis' father and his friends, Paul sets out to find Jeff. Finding the dismembered body of Grimm and discovering early signs that he is infected, Paul takes the group from the convenience store's truck and hits a deer, covering him in blood. He later reunites with Deputy Winston, who is partying with underage drinkers. After Winston is radioed about several infected people in a cabin going on a killing spree, Paul attacks and infects several of Winston's friends before knocking Winston out. Paul is later picked up by a passing truck and dropped off at a hospital. There, he is interrogated about the virus, but he cannot provide any response. The sheriff tells Winston to "take care" of Paul. Paul tries to warn Winston about the drinking water by saying "water..." but Winston only responds by dumping him at the edge of a creek.
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 later becomes ecstatic upon realizing he is the only one who made it without being infected. As he raises his arms in victory, he is gunned down and killed by several police officers, who burn his body along with the others. Back at the convenience store, a couple of children sell lemonade, that they have made with water from the creek Paul was dumped in, to the same police officers. In addition, a large truck, filled with bottles of water taken from the creek, can be seen leaving the store.

Question: Who developed infected sores on their back during sex?
Answer: Marcy