I was given this level 100 Mew in a Master Ball years ago by a friend of mine, and I finally got around to transporting all of my Pokemon to my X version. Unfortunately, Mew was the only one who did not transfer. I thought something was up because I had already transferred some event Pokemon, tons of legendaries, and shinies. However, Mew has (or at least HAD) the Pokerus. The little smiley face is still on his summary page.
My initial thought is that it is either Doom Desire that won't transfer for some reason or more likely, it is the Pokerus. Any ideas?