Mikuru became a centauress.

Realizing what she had done, Haruhi tried to give Mikuru a comforting smile. "I'm sorry, Mikuru, I completely forgot about the Virus!" Before Mikuru could ask her what she was talking about, she felt a strange heat in her lower body, and looked down to see her legs becoming covered in a short layer of fur. As that was happening, her skirt and panties started feeling tight, until finally her ass burst out of her clothes as it grew out behind her. Mikuru let out a squeak of fear and covered her eyes, unable to watch it any more. Haruhi, however, had no such issue, and watched the transformation with awe. The transformation was quick, taking only a few minutes, but when it was done Mikuru looked entirely normally from the waist up.

However, just where her torso would normally meet her hips, her body transitioned into that of a horse. Mikuru's horse body had a soft brown coat, and the addition of it added several feet in height to her. Her horse legs were strong and muscular, and Haruhi walked over to her behind, where a tail was swishing around, so admire her horse ass and horse pussy. Apparently not content to just stare, she reached out a blue finger and stuck in inside Mikuru's pussy, admiring its size. "M-Miss Suzumiya!" Mikuru cried. "Please don't t-touch me there!" Tears started falling down Mikuru's face as the truth hit her. "I can't believe I caught the Monster Girl Virus..."

Now that Haruhi has 'seen proof' of the realness of the Monster Girl Virus, reality altered even further. It was now a known, recent condition, and a small portion of the population were now monster girls. No one knew where it came from, and despite their best efforts, no one had been able to reverse the transformation. Haruhi did not know any of that, having just assumed it was real herself, but this explained both Mikuru's reaction...and that of another member of the SOS Brigade who entered at that exact moment to the bizarre scene of a succubus fingering a centauress.