The thing was that in the wish-altered reality, the change virus had been around for over forty years, and it was beginning to have repercussions beyond all the obvious societal adaptations. Since humans could only be born from a human mother, and the human mothers past the first generation exposed to the virus generally had contracted it themselves and avoided changing, this meant that resistance to the virus was gradually being bred into the human gene pool, theoretically leading to a world in which everybody was immune to its effects.
Of course, that was a long way off, if it happened at all. But what it had resulted in was a current generation that was beginning to resist or delay contraction of the virus. The "first eight years" statistic was still generally true, but it was becoming less accurate, as the next census would show. Some children were avoiding contracting the virus at all until later in life, when they might or might not change.
All of which is a roundabout way of saying that, though Erica had more or less resigned herself to remaining human, she had actually manage to resist contracting the virus on her previous exposures. And if she had given Mikey some personal space, that might still be the case. But she hadn't, and it wasn't. As active as Erica had been around the new demon-girl, she might as well have been huffing concentrated virus from a paper bag (which she actually might have done, given the option.) She had definitely contracted it, and while there was no known link between severity of exposure and likelihood of transformation, it probably wasn't helping things.
The first thing that changed were her legs. As she slept, they began to darken and shift. The surface changed from her soft human skin to a black, glossy chitin; they changed proportion and structure as well, the two halves of each leg becoming hollow tubes, and the foot a tapering cone shape about as long as the other two segments, tipped with a pair of small claws, between which grew many microscopic bristles, enabling them to cling to just about any surface to some degree.
As this change was finishing, Erica's buttocks began to be covered in chitin as well, as they extended out behind her, forming a new section of her body. From the pelvis up, she was as human as ever, but below that she was rapidly becoming something else entirely. Three more pairs of identical legs grew out from the new segment, while yet another, much larger segment began to grow out from the back of that. At the tip of this formed a set of spinnerettes, connected to silk glands inside the new abdominal segment.
When the change was finished, a young drider-girl slept contentedly in her bed, completely unaware of the discovery that awaited her in the morning.
