They're zombies.
Maybe this joke should have been: In a zombie apocalypse how can you tell the difference between the zombies and the Fibro's? You don't have to:
Fibro's are the first one's getting bit by the zombies so. Fibro=zombie
Do you think a Fibro zombie would be slower or would it be faster since it can't feel pain?
Zombie life is starting to have some appeal.