Foster's Home for Imaginary Friends (2004)

S3E9 Land of the Flea

Eduardo finds himself overrun by fleas, but instead of being bothered, he develops a friendship with them. Meanwhile, Frankie and Mr. Herriman are determined to eliminate the pesky critters from the house. As tensions rise between Eduardo’s newfound companions and his friends, a humorous conflict ensues, highlighting the challenges of friendship and acceptance.