Five years ago there was a disease that spread quickly. The disease was known as Celius, and it was very brutal. Killing off thousands of ponies. It was successfully eradicated, thanks to the help of Twilight Sparkle, and her slightly demented scientist friend.

But now, five years later, a new disease comes to life. One only the contents of a single book would know about. One that could may or may not be more dangerous than Celius.

Because this one isn't a virus.
It's a worm.