Smellfungus - Someone who is forever the pessimist or drags everyone down.
Let's use it in a sentence, shall we?:
When the mood of the party took a nosedive, Eloise said, "I think there is a smellfungus among us." To which Michael replied, "Either that or we are about to witness the apocalypse."
Your turn! Use smellfungus in a sentence via comments.