What could be worse than getting diagnosed with one of those? How about having a doctor stand over your death bed, shrug his shoulders and say, "I dunno, magic?" After all, nothing is scarier than the unknown, and science can't explain the first thing about some pretty horrific diseases, and even less about how to stop them.
Firmly in the category of "things somehow made more terrifying by a ridiculous name," the Dancing Plague was an actual disease that killed people. In 1518, in Strasbourg, France, Frau Troffea started dancing in the street. After six days, others began to join in; after a week there were 34. By the end of the month there were 400, though at that point most of the people started dropping dead of exhaustion, starvation and strokes. From dancing.
Deadlier than a crocodile with rabies and a machine gun.
Now, you may be thinking that "tackling and forcibly stopping" the people who were literally dancing themselves to death may be a sound idea for preventing the afflicted from dying. But of course, at at time when there was no such thing as Hazmat suits, that would have taken an enormous set of balls. Instead, Strasbourg officials had the brilliant idea of getting everyone to dance more--they herded the afflicted indoors, built them a stage and paid minstrels to crank out more jams, which eventually resulted in most of them dying. Clearly this was the pinnacle of 16th century medicine.
"Well, they're not dancing anymore, are they?"
The whole thing just kind of ended and, despite almost five intervening centuries, modern medicine has no explanation for why 400 French people suddenly danced themselves to death. Many theories have been offered, such as ergotism (poisoning by a certain type of fungus) and mass psychogenic illness, but they have some issues.
MPI is the first runner-up for the most plausible explanation, but it would have required 400 people to all develop the exact same "mass hysteria" of dancing at a staggered pace over a month, which is pretty unlikely. In the case of ergot poisoning, one of the common side effects is loss of muscular control, which makes complex movements (like dancing) impossible.
Then again, the only alternative seems to be demonic possession or witchcraft, so maybe we'll just go with the fungus thing and pretend it never happened.
5Stiff Person Syndrome
Not everything terrible in the natural world kills you. Sometimes it just pounds your crotch into pixie dust with a meat tenderizer (figuratively).
Nature is a lot like Vinnie Jones.
Stiff person syndrome is one of those cases. People afflicted with it experience increasingly progressive rigidity, so it's kind of like a muscle cramp that never, ever goes away. As the disease progresses, the victim's muscles become more and more stiff until they are completely paralyzed--the muscles becoming so constricted that they are frozen. In severe cases, the condition results in difficulty breathing, problems swallowing, muscle ruptures and fucking broken bones.
We have no idea why this syndrome develops in some people. It might have something to do with having diabetes; it could be an autoimmune disease; and it could be the result of a mutated gene, which would be the dumbest mutant power ever conceived.
"You can just take that shit right the fuck outside."
This brings up the very real possibility that you could have it right now and not even know it. There is no way to predict that it will happen to someone, or how long it will take to cripple them once it starts. There are some treatment options, most of which involve lots of injections that relieve part of the stiffness. The bottom line, though, is your ass is wheelchair-bound regardless of what you do.