Cancer of the bum. Otherwise known as Colon cancer, or in general cases, any bum related injury. Piercing a bum cancer tumour can lead to potentially fatal bum AIDS.
Katie: I think I have bum cancer. My ass is bleeding.
Me: You do not have the bum cancer, you ass.
Katie: I hurt my ass:(
Me: What's that I hear? BUM CANCER?! You have bum cancer?! Ergh.