Pre-Infection, the children were presumably normal humans. This all changes after a tainted batch of chicken nuggets from Happy Poultry Farms is delivered across North America during the summer of 2014. One day in Fort Chicken, Illinois (where the film is set) a fourth grader named Shelley Linker inadvertently infects herself with the virus through her lunch. Throughout the day, Shelly exhibits signs of infection, such as blisters and a runny nose.
After constantly being bullied by Patriot in class, Shelly infects him with the virus and escapes to the playground, where she tries to dig an exit route under a fence. Before Shelly can finish however, the rest of the school is dismissed to recess, creating potential victims of the virus. Dink, another one of Shelly's tormentors, verbally assaults her. Shelly quickly infects Dink and returns to her work. Dink then sprints around the playground, slashing many children with his nails and infecting them with virus, catching the attention of Mr. Pederson and Ms. Nash. The two teachers attempt to work things out with the newly infected children, but they are quickly dispatched. Two children, Tamra and unnamed student are infected offscreen, but they don't transform, which prompts the children to kill the unnamed student while Tamra hides in a tree. Patriot mauls the nurse who was tending to his wounds as Vice Principal Simms is torn apart. Wade Johnson, the school's PE administrator, who had been too busy shooting hoops to notice the Cootie Kids, makes a break for the school's entrance, injuring a few infected children in the process. Wade, after locking the children out, makes his way to the surviving staff in the teacher's lounge. Meanwhile in the principal's office, Patriot, who has killed the secretary, destroys the teacher's cell phones and phone lines, which shows Cootie Kids still have some intelligence left.
The children notice the teachers through the window, but a police officer comes after being summoned by a teacher's rape alarm. Shelly (who has dug a large enough hole under the fence, escapes and quickly dispatches the police officer (will finish this later).
Before the virus, the Cootie Kids were probably normal children. When they contracted the virus, they became sadistic, brutal, monstrous, uncaring, aggressive, bloodthirsty, cannibalistic, and surprisingly gleeful about killing.
After gaining the virus, the children begin rotting and developing blisters and black veins all over their bodies. The kids are almost never seen without blood on them unless it was pre- infection.