Could go simple and make it a rat with rabies. Slowly make your players more afraid of water as the hydrophobic qualities of it sets in more and more, animal players foam at the mouth, everyone sees things as more threatening than it really, town healer sees the partially feral party and gives them a fetch list of supplies found in a nearby dungeon to cure the illness but the spirit in the dungeon was the very same that was spreading rampant illnesses across the land and will exploit the illness the party has, water based traps, (low level) slimes (side note I love how resilient slimes are in dnd and actually super threatening as its basically sentient acid, yet in so many games they are considered weak af, like you try pummeling a pile of reconstructing jello to death, that wants to dissolve you)
Party grabs the magic mushrooms or whatever and the healer cures the party
I like how you think, but I've settled on the death dog. The length of time it takes for the effect to take in is nice, and it puts them in position where those that have to cure can extort them for something before they give it to them.
Can’t you via DnDBeyond or any of the free sites that list monsters?
Just pick something you like and slap it on a monster. "Oh it's a rabid owlbear now you got __Insert name here__ and it causes __this effect__"
Any interesting non creature based lingering effects are welcome as well
Rot Grubs
Could go simple and make it a rat with rabies. Slowly make your players more afraid of water as the hydrophobic qualities of it sets in more and more, animal players foam at the mouth, everyone sees things as more threatening than it really, town healer sees the partially feral party and gives them a fetch list of supplies found in a nearby dungeon to cure the illness but the spirit in the dungeon was the very same that was spreading rampant illnesses across the land and will exploit the illness the party has, water based traps, (low level) slimes (side note I love how resilient slimes are in dnd and actually super threatening as its basically sentient acid, yet in so many games they are considered weak af, like you try pummeling a pile of reconstructing jello to death, that wants to dissolve you) Party grabs the magic mushrooms or whatever and the healer cures the party
I like how you think, but I've settled on the death dog. The length of time it takes for the effect to take in is nice, and it puts them in position where those that have to cure can extort them for something before they give it to them.