My guess:

It's a new virus or prion disease.

These 50 people probably all ate meat from the same cow in a hot dog 10 years ago. That hot dog or source cow will never be identified because humanity doesn't keep a sufficiently traceable record of such things, even though it would save lives to do so.

It probably isn't transmissible between people, so there will be a few hundred more cases of this over the next 20 years, and then it'll disappear.

>Aside from memory and mental issues, symptoms have included the wasting away of muscle, pain, and spasms.

That's terrifying.

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