The Outer Limits - Season 2 Episode 13 From Within

A mentally challenged boy named Howie is the last unaffected person in a small town overrun by a strange madness. Miners unearth ancient parasites, in the shape of worms, that attack the brains of their hosts. While the infected townsfolk lose all their inhibitions, Howie must save his sister Sheila, the only person who truly cares for him. Deprived of Sheila's guidance for the first time in his life, Howie struggles to evade his maddened neighbors and destroy the parasites. In the process, he becomes a hero to the whole town.