Full Moon Fever is a novella written by R. L. Stine. It was first published in 1999 and is the 22nd book in the Goosebumps Series 2000 book series.
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Robbie and his sister love trick-or-treating--even at scary old Mrs. Eakins' house. But when they both gobble down one of her "Beast" candy bars, things start to get very hairy.