Code Lyoko - Season 1 Episode 17
Amnesia

When learning about NanoDoctors, small robots being designed to cure dieases, Ulrich holds some gel that the NanoDoctors would live in. Od pranks him and pushes the gel into Ulrichs face, and some minutes later Ulrich suddenly faints and loses his memory. Strangely, amnesia spreads throughout the school, and Jeremy and Od think its Xana at work. Yumi and Od are virtualized while Ulrich escapes from Sissy, whos trying to convince him theyre in love. Ulrich gets virtualized and re-learns how to fight from Od. They help Aileta deactivate the tower. It ends up being the NanoDoctors gel habitat was infested by Xana Nano-Virii and got up Ulrichs nose.