Wednesday’s Wonderful World Of Wikipedia: Fictional Diseases & Toxins
This is an exhaustive list of nonexistent, named medical conditions which appear in fiction which includes diseases that have evolved naturally, artificially engineered biological weapons, hereditary diseases, magical diseases, and technology-based diseases. I think my favorite is Primary Vocabularyitis, from a episode of Bewitched titled “Samantha the Bard”, that causes Samantha Stephens to speak in rhyme.
As a bonus, here is a list of fictional toxins.