A.N.: This story was inspired by 2 things: the Harry Potter wiki article on Dementors (the 'behind the scenes' part), and the fanfic story Witness by Quazi Jo. This is a crack fic. Don't take it seriously.
Harry and Dudley sat talking, finally mending their relationship after 18 years of animosity. They'd talked about the war and future plans, and there was a lull in the conversation. Suddenly, a thought occurred to Harry. "Hey, Dud, you never told me what you saw when the Dementor attacked you?" he asked out of the blue.
Dudley shrugged. "Not much. Didn't really have much happiness to suck. Just noticed what loveless, great, bullying prat I was." Harry nodded, and there was a moment of silence. "What are dementors anyway? Where to they come from?"
"Nobody really knows. There's lots of theories, though," Harry answered.
"That's not entirely true. We have studied the remains of dementors who starved to death.," a voice from behind Harry cut in. A guy their own age with short brown hair walked up to them. "Sorry, I didn't mean to eavesdrop, but I overheard, and have a vast knowledge of plants," he said.
"That's...interesting," Dudley said, not sure what that had to do with anything.
"This is Nevil Longbottom, he's a friend from school," Harry said.
"I'm hoping to work there after I graduate," Nevil said.
"Congratulations. So what have they found out about dementors?" Harry asked.
"Well, we know that the physiology of their bodies is exactly the same as humans, except for strange tissues that are found throughout the body but don't seem to have anything to do with any of the organs, and don't seem to form a separate organ. It seems to be similar to a fungus. We also know they appear in areas with a lot of rot and decay. It can only infect those who have no love or happiness, because those feelings make white magic. So it's ironic, isn't it, that it's only food source is love and happiness? And souls, of course," Nevil explained.
"Wait a minute. You mean all this trouble was caused by a bloody mushroom?" Dudley exclaimed. Harry and Nevil both laughed at this.
"Yes, I guess you could say there's a 'fungus amongus,'" Nevil joked.
"Well, that's one magic mushroom I definitely DON'T want!" Dudley exclaimed.
"Yea, it'd give you Hell's own athlete's foot!" Harry gasped, still giggling.
"Or a bad case of wetrot," Nevil said.
"I guess you could say their spoiled!" Harry giggled.
"I'll never eat mushrooms again!" Dudley said. "I don't want no moldy doom!" He then started singing:
A terrible death to die,
A terrible death to die,
A terrible death to be molded to death,
A terrible death to die.
By this time, they'd run out of both breath and punchlines, so they paid the waitress and went home.