A pure geometric form, the spiral, functions as the vector of corruption: it is not a creature or disease but a pattern that infects by being perceived and reproduced in bodies, objects, and behavior. The horror derives from a meaningless shape acquiring agency, so that cognition and attention themselves become the channel of harm.
A man becomes obsessed with spirals, collecting them and finally contorting his own body into a spiral shape, cramming himself into a circular washtub where he dies twisted beyond human proportion. His corpse is later cremated and the smoke itself rises in a spiral over the town, spreading the obsession to his widow.
The town of Kurouzu-cho is bounded so that escape routes loop travelers back, making the affliction a property of place rather than of individuals. Viewing catastrophe as spatially sealed reframes the inhabitants' choices as irrelevant: no personal virtue or flight alters an outcome fixed by where one stands.