Colony

The ending turns on rules the film establishes ahead of time. Each colony is governed by a single dominant host that controls the rest of the infected: when soldiers shoot the infected monkey from the enclosure labeled 1, every other monkey instantly goes dormant. Kill the dominant host and the whole network freezes. The infected themselves are already lost. As the film states earlier, infection effectively destroys the nerves, and they cannot be healed or regenerated, so nothing can be done for anyone already taken; a vaccine would only inoculate those not yet infected. But Seo Young-cheol, the man who released the virus and now commands the hive, is also immune, which makes his blood the only source for that vaccine. Killing him stops the outbreak and destroys the only protection the uninfected would ever have. The government will not carry that burden, so Kwon Se-jeong does. She beats the hive with the ant mill she described earlier in the film. Army ants separated from their colony will follow each other's pheromone trail in a circle until they die. Se-jeong walks out wearing a mycelium-coated coat stripped from an infected corpse, so the horde reads her as one of its own, and when Young-cheol orders the entire colony to attack her, she throws the coat over one of the infected. The horde is now commanded to destroy something it perceives as itself, locks into a circular pursuit, and the feedback loop crashes the shared network. Yeon Sang-ho has said in interviews that he researched collective intelligence while writing the script and learned of the real ant-mill phenomenon, first observed by the entomologist William Morton Wheeler. The crash resets the infected to their day-one state, when they could not tell a real person from a picture of one. Se-jeong lures the reverted horde at a bus advertisement showing human figures and slides underneath the bus. In the surge, Young-cheol is shoved into a burning car and dies there, killed by the new humanity he engineered. The moment he dies, every infected in the city freezes where it stands, and the vaccine dies with him. We tag the ending bittersweet plus ambiguous, and the film earns both. Se-jeong and Kong Seol-hee survive and resolve to develop a cure themselves, but everyone Se-jeong fought to protect inside the building is gone, and Seoul is left full of frozen infected no one can heal. Then the sting: as Se-jeong walks away, one of the frozen infected moves on its own. The rule the whole ending rests on says a colony without its dominant host stays dormant, so an infected moving with nobody commanding it means the outbreak could restart with no kill switch left. There is no post-credits scene.