Haruhi thinks this is due to a Monster Girl Virus, which becomes real.

"Whoa!" Haruhi exclaimed, standing up from her chair and walking over to the clubroom mirror, staring at her undeniably transformed body. "How did this happen...it should be impossible..." As Koizumi had already suspected, at heart Haruhi was a rational person. But even a rational person would find themselves jumping to impossible explanations after transforming into a monster, and something from the website that started all of this jumped out to Haruhi. "Wasn't there something about a Monster Girl Virus...wow!" she shouted, smiling like the sun once more. "I was feeling kind of sick this morning, wasn't I?" In fact, she'd only sneezed once, while starting at the sun on her walk to school, but her memory tricked itself, fabricating a morning of aches and a mild fever.

The door opened quietly, and Mikuru entered timidly, clearly unaware her erstwhile tormenter was already in the room. Then she noticed the succubus in a sailor uniform standing in front of the mirror and froze. "M-Ms. Suzumiya?" The time-traveler was baffled. What had the focal point of a massive time-quake done now?

For a reason even Haruhi couldn't understand, she found herself staring into Mikuru's eyes and walking towards her. The upperclassman couldn't move, like a deer frozen in headlights, and Haruhi smiled as she reached her, then leaned in for a kiss. Internally, Haruhi was confused. Not exactly upset, just confused. She was pansexual, so Mikuru's gender wasn't an issue, but while her body had always been sexy as hell, her personality had never been actually worth the trouble of getting involved, at least to Haruhi. Not that any of that interefered with the kiss, which Haruhi was quite enjoying, and by the way Mikuru was blushing, her mouth agape as Haruhi pulled away, she wasn't the only one. Then, she remembered: she had the Virus! Of course, no such virus existed, but Haruhi believed it did, and so it suddenly did...and Mikuru had caught it.