Since the idea was his own and excited him so much, Ty figured he ought to be the one to first get the virus he'd come up with. His wish made it come into existence, and he practically felt it as it formed in his body.
Ty undressed, wanting to see the ways it affected his body. By the time he was standing naked in front of his closet mirror, he already felt the first strains of warmth- it felt like an odd combination of a feverish rise in temperature and a hormonal rise in arousal.
The early signs of reassignment were already emerging- his face and cheeks looked softer, his lips were fuller and his jawline less sharp. He swallowed and felt his larynx slightly shrink, his voice's pitch increasing. All across his body the thin body hair of an adolescent male was either turning thinner still or receding into the skin altogether.
An involuntary shiver ran through him, and before his eyes his muscles slimmed, his arms and legs and hands and feet shrank. They clearly weren't childlike again, however, just... not a teenage boy's. A moment later, his figure followed suit, his shoulders narrowing, torso curving inwards and hips flaring outwards. His butt seemed larger, too, and despite the enormity to how his body was changing, Ty felt like he liked it. A lot. It felt like seeing a whole new perspective on something and realizing it was right.
Ty groaned in his higher-pitched voice as an arousing sensation ran through his chest and crotch, and it became undeniable to him what the new perspective was. He did not need to be a boy. His hands moved to his chest and rubbed his nipples, and he moaned eagerly at how sensitive they were. The nubs perked up, and as he breathed heavily large soft mounds of flesh sprouted around them gradually.
His breasts. No, he realized- her breasts. She would be a girl soon.
Realizing this made the last stage of her transformation possible. His balls moved up inside him, the flat mound adjusting to form labial lips, while his penis receded into her, growing more and more sensitive. While a womb and ovaries formed inside her, Ty's feminized hormones stimulated her to an aroused state. She imagined a handsome lover with a long thick cock, saw him fucking her new opening, fantasized about him playing with and sucking her tits. She was adjusting to her new form's sexual purpose- instead of having a body made to fuck, she had a body made to be fucked.
She watched in anticipation as the changes finished, her slit and the reproductive system within finishing forming and his penis finishing its shrinkage as her tiny, sensitive clitoris. Once her body had stopped changing, Ty looked at her new reflection and was amazed. She had become a girl, and she looked as if she had never been anything else. She felt very pretty.
Once she got done looking at her body, Ty wondered how to transmit the virus...