Page 232 — From the Zebra's Mouth

And the truth comes out.

Zecora: The Poison Joke cure has been my life’s work. For seven years now, I’ve been forced to lurk. I’m very close, but not there yet, so I need one last data set.
Fluttershy: Are you… also affected by Poison Joke, Zecora?
DM: She simply nods.
Pinkie Pie: What about all the info I got? That stuff about the ponies in Ponyville disappearing or going mad?
Zecora: Yes, those ponies had to endure the effects of my partial cure. Every pony I talked to would mock me or flee. So I conducted experiments secretly.
Twilight Sparkle: So you WERE using ponies as test subjects!
Pinkie Pie: I WAS RIGHT ALL ALONG!!