S1·E17 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 Ulrich's face, and some minutes later Ulrich suddenly faints and loses his memory. Strangely, amnesia spreads throughout the school, and Jeremy and Od think it's Xana at work. Yumi and Od are virtualized while Ulrich escapes from Sissy, who's trying to convince him they're in love. Ulrich gets virtualized and re-learns how to fight from Od. They help Aileta deactivate the tower. It ends up being the NanoDoctor's gel habitat was infested by Xana Nano-Virii and got up Ulrich's nose.