1) When someone's breath is SO rancid, you can't help but utter, "Yer breath is KILLIN' me (smalls)!!" Hence the term: Breath Cancer. Breath that Kills. It's no joke!
2) OR, when yer friend has a "thinko" as she tries to say 'Breast Cancer', and really she says, 'Breath Cancer'. Then you erupt in laughter cause it's just THAT funny!
Oh my Gawd, did he just eat a clove of garlic? He has total Breath Cancer!!!