Cooties

The mutant-virus infected chicken nugget is produced and shipped to Fort Chicken Elementary, kickstarting the outbreak. Shelly Linker, a fourth-grader, eats one tainted nugget and contracts the virus, becoming a carrier who can spread it to others.

Clint Hadson substitutes at the school, stepping in for a class. He is reunited with his former high school crush Lucy McCormick, who is dating the physical education teacher Wade Johnson.

During Clint's class, Shelly lashes out at Patriot after he tugs a pigtail from her scalp, igniting chaos in the classroom. Clint is scratched in the scuffle as Shelly flees, signaling the outbreak has begun to spread.

Patriot is taken to the nurse as Shelly lunges at him. Shelly infects Dink, who then scratches multiple children on the playground, spreading the virus and leading to the deaths of several staff members, including Mr. Pederson, VP Simms, and Sheriff Dave.

Clint, Lucy, Wade and the surviving staff flee the faculty lounge and seek safety in the library, where they are soon joined by Calvin, an uninfected student. The group later tentatively barricades themselves in the music room as the threat closes in.

Wade quarantines Clint after discovering scratches on him. Doug deduces the virus behaves differently for adults and children, and Calvin coins the term 'cooties' as the culprit behind the infection.

The staff plan to reach the roof to call for help, but the first arriving parent is killed by her infected child, underscoring the scale of the outbreak. Wade is forced to kill Dink when he follows them into the auditorium, and the group is joined by Tamra, a teenage scratch survivor.

Patriot cuts power, sending Calvin into diabetic shock. The group escapes with janitor Mr. Hatachi and takes refuge with him as the situation grows increasingly desperate.

Clint crawls through the ventilation system to obtain a chocolate bar for Calvin, and retrieves Wade's truck keys and the group’s cellphones. Lucy accompanies him, and they manage to bring the chocolate back to revive Calvin.

Clint and Lucy are separated from the others and end up trapped in the library, where they confess their feelings and kiss. Meanwhile Wade radios an apology to Lucy, and the group arm themselves with improvised weapons to fight the infected.

The group fights their way through the hallways and parking lot as the campus is overrun. Patriot, hiding in Wade’s truck bed, attacks Clint and is crushed against a tree when the truck moves.

The survivors push on toward the nearby town of Danville, only to find it overrun as the viral outbreak has spread across the country. They realize help is scarce and the world is changing as the infection expands.

Inside a children's entertainment building, they retrieve a contaminated nugget for study in hopes of a vaccine, but Shelly and the infected children close in. Wade and Hatachi arrive to help, and they burn the building and escape, driving toward Antarctica.

In the post-credits scene, Hatachi sits at the school, finishing a story he was telling the teachers earlier. The scene hints at what might come after the events shown.