Dnd session based off of hanahaki syndrome (not the song specifically but it got my creativity going) but with a twist
Depending on how much your party knows about the disease, you can even make this a whole 2 session thing (also depends on how long your sessions are. mine are usually around 2-2.5 hours long but you could def get this whole quest in one session if yours are longer) where your party has to find out what the disease is. For my campaign there's an area specifically made worshipping two gods of life and the elements so I may have a whole village that knows what the disease is but you could make it hard by having one or two books mentioning the disease and needing to have your party piece together clues and cures
In between quests, or perhaps just traveling to another quest area, the party stops at this town to stock up, rest, make some extra money. As they're doing that they meet a character who helps them find/do/make/whatever and somehow it gets out that they have hanahaki, albeit they don't know. They have all the symptoms, flowers, thorns, coughing fits that won't quit. The party has to find who their unrequieted love is. The only problem is that the character has some type of memory loss. They can't remember the past few months of their life and we're on their way to someone who could help them when they ran into the party.
The party helps them get to the person who basically tells them to go to spots they visited often or had important meanings to them. Around 3-5 areas should be enough for the quest. During the last quest the characters unrequieted love is actually found out to be the bbe who knew of the characters love but thought that it would be a mercy to wipe their memory. During the quest the disease gets worse until the party has to carry the character, thorns wrapping down their elbows, torso, knees and creating thick stems around their limbs. Flower blossoms bloom in their hair and petals falling out of their mouth with each word. The character does regain the memories and remembers everything about their love, nector tears falling as they explain how they knew the bbe was evil but they were willing to help them as long as they stayed with the character.
You could take this in a few different directions. The character could beg the party to kill them, they could sucumbs to the disease and become a giant flower bush, or they could become a type of plant monster.
If you choose to have them become a monster/a flower bush, you could be evil and have them come back the next time the party fights the bbe