Your Celebi is probably Legit.
The Pokémon Bank Celebi is available worldwide to players that have accessed the Pokémon Bank application anywhere from the 25th of December 2013 to the 30th of September 2014.
Pokémon Bank was released in Korea on the 25th of December 2013.
You obtain Celebi at Level 10 inside a Luxury Ball and it receives a random nature.
My guess is that the player who your Celebi used to belong to has levelled it up from Level 10 to Level 42, changed it's move set and then infected it with Pokérus.
Hope I helped :)