As I reached the top of the stairs, I came across Fast slumped against a wall, panting. As soon as I rounded the stairs into her sight, she turned towards me. "There you are. Why are you so slow? I thought you were chasing me?" She waggled her purple flank. "Or maybe you don't like Twilight? Maybe something else?" She became a male pegasus with rainbow mane and cyan fur. With a grin, she changed as if she were made of water. "Something athletic and powerful, perhaps?"
I held out a flat hand. "Stop right there, Fast. This isn't you. You have a herd, and I imagine you love them, and going off with me would hurt them. You don't want that."
Fast paused a moment before a frown spread over her cyan snout. "You're right. I'm regressing. Buck it all, I'm better than this." She moved to brush past me. "I'm going to make Twilight scream for a little while, don't mind me."
While that might normally have been a victory, I quickly realized what would happen if she tackled Twilight. I wrapped magic around her quickly and hefted her up. "Stop. You're dangerous right now."
She snarled and spread her wings. She was no slouch with magic, and easily shattered my grip on her with a flash of her suddenly-returned horn. "Hey, thanks for snapping me out of it, but keeping me from Twilight's a one-way ticket to painville. I'm going to be with her, and you won't stop me."
Cindy and Sandra came around the corner as Fast vanished in a sparkle of magic. Sandra scowled. "Where did she go?"
I had a pretty good idea. "Basement, hurry, she's going to infect Twilight in a moment." I hopped through the void and landed at the base of the stairs. Surprisingly, she was above me. She hadn't hopped all the way down. Maybe she wasn't as practiced with teleporting? "Fast, please. This is very important, or you're going to hurt Twilight and everyone you love."
Fast, now appearing as a brown-furred stallion with a silver mane that I didn't recognize, glared at me over the side of the stairs. "You keep bringing that up! If I go with you, they're hurt. If I don't go with you, they're hurt." She squirmed in place with a little grunt. "Stop messing with me already. I know you're a guest, but you're really stressing me out. Start talking straight, right now."
Cindy appeared beside me and grasped my hand in one of her own. Our power swelled instantly along with a feeling of completion. "Fast, we're sorry. This is our fault, but you're infected, and you could spread that to Twilight, or anyone you touch besides our herd. We need to clean you before you do that."
Fast leapt suddenly, spreading wings she hadn't had a moment before and soaring to land just in front of us. "Fine! I don't... I don't even care anymore. If you won't let me have them, then I'll have you. Twilight and Night will forgive me. They'll understand..." Frustrated tears were escaping from her eyes. "Just make it stop hurting..."
Neon appeared, galloping down the stairs with Sandra and Karen right behind her. Fast darted her head back to face them, then scowled at Cindy and I. "What's it going to be? Decide quick."
Thinking quickly, I sat on my haunches and offered my free hand. "Come here. We won't let you hurt anymore."
"What's going on here?" A new presence was felt. Night Watch, the bat pony with a foal, peered at us from her perch above us. "Fast, down girl, you know better than that. William, the rest of you, don't encourage her."
Fast let out a keen wail of dismay and vanished, skipping to the door of Twilight's library and throwing open the doors. "Twilight! I need you!" She had taken a new form, of a unicorn with pale looking fur and oddly malfunctioning hind legs. Despite her apparently crippled state, she hobbled and dragged herself forward with obvious desperation.
I released Cindy's hand and trusted in her as I jumped between space to be in front of Fast and catch her. I grabbed her in careful hands and held her close as she began to sob. Her body was burning up with a fever, and her eyes looked wild and unfocused. She was just like those first humans, leaping on random people and french kissing them to infect them, raping and doing anything they could to spread the disease as a living vector for it. It had clearly consumed her thoughts and she was in misery.
Twilight looked up from one of her machines. "What in Equestria is going on?" She vanished and appeared beside me. "Is Fast alright? What's wrong?"
Fast surged out of my grip and jumped for Twilight. I put up a hasty shield that she bounced off of roughly in her haste. Blood seeped from her abused snout. "Twilight... please, I need you..."
Twilight's wings went wide. "William! Let her go right now!" She clopped a hoof down and magic rolled out of her in an intense wave stronger than I'd seen from any other pony before. She truly was a princess. It evaporated my shield like tissue trying to stop a hurricane. "This is unacceptable! I exp--" her words were cut off as Fast tackled her to the ground. I tried to separate them. I felt Cindy try as well, but our magic had been entirely cut off by that potent wave.
Fast pressed her snout against Twilights and they kissed. She kissed with all the wild desperation she felt, invading Twilight's snout and wrestling one tongue against the other. I could feel the gift spreading quickly into Twilight, making her glow in my secondary sight. Twilight had blushed with confused embarrassment at first, but it quickly turned into equal passion, and the two were lost to the world.
Night tilted her head from up above. "Oh." It was as if she understood all that had happened without needing further words. "Is this something you brought with you?"
Sandra nodded at Night. "It's safe for us, but not for you, or any other Equestrian. It spreads through fluids only. Skin contact's not enough."
Night quickly nodded. "I'll tell the others." She spread her wings and launched off towards the upstairs to warn the rest of the inhabitants of the tower. "Keep them here!"
I looked to Twilight and Fast as Fast wrestled Twilight to lay flat on her back and moved to slide up on her, belly-to-belly, sporting a thickness that belonged on a stallion, not her mare body. Keeping them there didn't seem like a difficult task, all things considered. Fast had lost all focus on her form, and her attempts were being thwarted by her lame legs, making the two of them moan in frustration as she struggled to get into proper position.
Cindy closed the door to the lab, separating us from the rest of the herd, but they could guard the outside while we kept an eye on the inside. She sat beside me and sighed. "This is really bad. Should we try to clean one of them out now?"
I wasn't sure. "It could hurt them. We've never tried manipulating the gift in a natural pony before."
Cindy nodded. "Yes, I know, but what's the alternative? She's one of their princesses, of all people. We can't have her being infected and wild. You felt that magic of hers. She could have this whole world turned in no time if she put her mind to it."