thanks for the update :)
now time for me to ramble. the teacher talking about convergent evolution made me think about succubi naturally evolving. at first i thought about some divergent form of hominid that evolved to be lewd, but the closest thing to that is a bonobo and it doesn't explain the reliance on human touch. then the thought of convergent evolution came to mind like maybe a fungus or a plant evolved human traits to better fit a similar enviorment, but that doesn't work because humans, plants, and fungi have different requirements to live. then i thought about mimicry. maybe a plant or fungus started to mimic humans to get nutrients from meat milk? i was satisfued with that, but i was reminded of viruses. that put a theory in my head that kind of matches the story's plot: a genetic virus that's stopped being viral through contact. let me explain. viruses can actually survive in dna and be passed down to our offspring (PBS Eons did a video on it if you're curious it's on youtube). what if a virus made it so it restricts nutrient flow in the bloodstream until it's made contact with another human? we know this could exist because of rabies and to some extent hookworms. but then wouldn't that just kill the human infected if they happened to be bot near anyone? yes, but when viruses get imbued with out dna, not all of it survives. the succubi with the stronger viruses are put out of the genepool. this also culls the hyper-viral ones that spread too quickly and killed everyone. this leaves us with modern succubi that need to touch humans, but don't spread the disease by means other than genetic exchanging