Go Eat Worms! (Classic Goosebumps #38) by R. L. Stine

They're creepy and they're crawly -- they're totally disgusting!

Todd loves his basement worm farm. Worms are fascinating creatures--it's just a convenient bonus that they happen to gross out his family. But when Todd starts to find worms in unlikely and unwanted places, like his bed, his toothbrush, and his spaghetti--the tables have turned. Now Todd is the one running scared!