Viral

Stacey and Emma move to a California suburb where their father has taken a job teaching high school science. Emma struggles to fit in, while Stacey quickly makes new friends and even starts dating CJ. The move sets up a tense, new routine for the family.

Emma befriends her neighbor Evan. Gracie, Emma's new best friend at school, begins displaying unusual behavior, eating constantly and coughing up blood. Gracie collapses outside the school and vomits blood on a boy while Emma goes to fetch help.

Newscasts report a worm flu spreading through the area. Emma's father travels to fetch their mother at the airport, but the government imposes a quarantine on the whole town, forcing them to stay inside. The family becomes isolated as outside help is cut off.

Stacey drags Emma to a party where she discovers CJ having sex with another student. The same party is disrupted when the ill student from earlier returns, coughing blood and attacking partygoers. Evan saves Emma from infection as chaos erupts.

The next day, Stacey develops a growth at the base of her neck and becomes ravenously hungry, signaling parasite infection. The family watches the strange changes unfold as the quarantine tightens. Her behavior signals the danger creeping through the house.

That night, Emma and Stacey observe their neighbor Mr. Toomey firing a flare gun as the military arrives. The soldiers detain Toomey and escort his infected wife away, marking the town's shift to a controlled lockdown. The environment becomes tense and dangerous.

Stacey confesses that their parents' problems stem from her catching their father with a student and telling their mother. She explains that nobody wanted to tell Emma, fearing she couldn't handle it. The revelation adds emotional weight to the already dangerous situation.

Evan arrives begging to be let in; Bill, Evan's infected father, breaks in. They use Bill's poor vision to their advantage, and Stacey shoots Bill, saving Emma, but then Stacey suffers another seizure. The family defense becomes a desperate fight for survival.

Emma and Evan lock Stacey in the bathroom to hide her from the military. Emma, recalling lessons from her father's science class, tries to remove Stacey's parasite, while CJ arrives and Stacey drags him in, beginning to devour his arm. CJ is found dead with his arm ripped off; Stacey tearfully apologizes, saying, 'It made me do it.'

Emma uses drugs to knock Stacey out and begins to extract the parasite, but Stacey awakens and pulls the parasite out herself. The sisters' attempt to cure the infection backfires in a devastating way. The family's fragile bond fractures under the weight of fear and infection.

The next day, radio announces an evacuation for those outside the quarantine zone. Evan and Emma discover Stacey left the house with a stash of food and pursue her, finding a large group of infected. Stacey reveals she can hear the infected as the parasite takes full control, forcing Emma to shoot her sister.

Emma and Evan escape through a window as the military bombs the area. They reach a gas station and find a photo of the family with a note on the back that says to meet their parents at their uncle's residence in Washington State. Evan takes a vacant car from the station, and the two depart for Washington.

Evan and Emma drive off toward Washington to reunite with their parents, following the note and the hope of safety beyond the quarantine. The journey marks a desperate move to salvage what remains of their family. The final image is of them driving away, away from the burning town.