Ok... I improv-ed this of the cuff in my saga, loosely based on some old White Wolf Dark Age Vampire power, and the idea of memes, and something that I thought felt vaguely flavorful.
But can this be designed? Msr. Fruny seems doubtful.
The spell: range is voice, but I envision it as very short range - it's whispered in the target's ear.
Target: Group, but starting off individual?
Duration: Negotiable?
Here's the effect: the caster whispers a riddle in the ears of the target. The target's mind is consumed by this riddle, and all they can do is try to mentally solve it as they speak the riddle over and over.
Anyone who hears the riddle also gets drawn into it and is effected by the spell, and so it spreads like a rumor or a bad idea. Deafness protects someone from it.
Possibly a high roll in Logic might allow someone to escape the effects the riddle.
Now, given all the above, and considering all the Merinita new durations, etc. from Mystery Cults, and given that some spells bend the rules of Hermetic magic due to their origins... can this spell be made?
As I pointed out, 3rd and 4th edition had "Catching Laughter of the Fay" (4th ed: ReCo 20, Faerie), but I don't think the spell is feasible in 5th edition.
This seems a little off for simple Hermetic Magic to me, but I tend to be fairly conservative in these things. I could see some Mystery Cult or virtuous magic (or Hedge Magery) being able to do it, though.
The original riddle effecting the original person could be done with a mentem spell (perhaps with a special duration) but to make the riddlde spread you'd need to have it target new people. I don't think that hermetic magic has a tool to do that.
You'd need to have a way to enchant the riddlle as something akin to an enchanted device that triggers whenever the riddle is told.
I could imagine a breakthrough or mystery cult initiation that allows enchanting of memes, but I wouldn't feel comforatble just asigning a special duration and a special target to a mentem spell and letting it go using vanilla hermetic magic (or even a researched new target catagory).
I'm thinking you might be able to almost pull it off with Animae Magic, creating a faerie in the mind of your target. It would still only affect one person at a time, but could arguably jump from one to another as they answer the riddle.