The Ick is a rare fish disease.
Bea faked getting the disease so she could escape getting eaten by hamsters while on the TV show I Dares Ya!. ("Hooray for Hamsterwood")
Milo and Oscar faked getting the disease so they could get out of doing a gym drill. Koi and Finberley later got the disease and couldn't make it to theater practise. (Gym Class Getaway)