Mutant Lice are giant hair eating lice and the titular main villains in the episode the "Operation: L.I.C.E." of Codename: Kids Next Door.
The Delightful Children From Down the Lane switched Numbuh 5's hat with a hat infected with the mutant lice when she was on a mission. From there, she returned to the Sector V Treehouse, where the louse escaped and proceeded to infect the entire Treehouse. By more accident, Sector V finds out that their weakness is nacho cheese dip and sets off to kill off the infestation before it takes down the members of the sector completely.
As the episode progresses, members of Sector V are taken down by the mutant lice, up until Numbuh 5 finally makes it to the Cheese Repository in the heart of the Treehouse with Numbuh 1. It is there that she meets the mutant lice's queen, who is several times as big as a normal louse and is purple as opposed to white. She defeats it, despite her braid being caught by the queen, by flooding the treehouse with cheese. In the end of the episode, they infect the Delightful Children.
- The lice, and the way the first louse burst out of Numbuh 5's hat, might be a reference to the Alien movies. In fact, the whole episode might have more than one Alien reference.