This episode aired back in 2006, so it's been quite some time since Ive seen it. But if memory serves it's about a smelly monster, and a joke gets made about one of the characters (a side char I think) who can't smell him because he is sick and sneezing a lot. I haven't had any luck finding it.
I think their might also be something in the Tracey Mcbean episode "Dirt Magnet" related to allergies, or something in Pet Squad's "drop in sometime" but I'm not too sure about those.