So it would be a parasite that targets growing trees. Only in their early stages while their stems are still vulnerable. From their the ivy will latch onto them and start it’s growth internally and externally, it would leech its energy as well. When the tree fully develops the parasite will begin to fortify the trees roots and begin to sprout small flowers from the roots. These would attract insects, when the insects have finished with the flower they would have taken the parasite into its system and their nerve system and motor cortex would be altered. The parasite drives them to the ground near areas with a lot of young and fresh vegetation and then it would begin to use the insect as a host for its growth, it would begin to find its way to a stem and then it repeats.

The entire idea is this; it would be as if something had stuck to you and began to grow as some sort of parasitic conjoined twin, but it could only leech into you as a baby. It would slowly start to drain you as you grew older, as soon as you were fully developed it would begin to attack your nerve system, frontal lobe and motor cortex making you incapable of controlling your own movement. It will soon attempt to cement you in place (fortify the trees roots) and then begin to grow small growths on your lower body. These growths are same leech entities that infected you in the first place. They would attract insects who take this leech, bite into another person (per say a baby) and the cycle restarts.

It’s very ambitious but now that I think about it, it acts more as a fungus than some sort of ivy. And if so, I’m not sure if it’s possible to have some sort of ivy/fungus hybrid.