Jeff thought that everybody should be able to enjoy this body. It felt so much better than he'd expected.
Plus, he needed an explanation for why he turned into Danny. He didn't want to actually take over Danny's life, he just wanted to live in the body.
He used the Chronivac to create a virus, for which he would be patient zero. Everyone exposed would turn, against their will, into the exact same body: a short, stocky, teenage boy with thick thighs, a handsome wideset face, a jiggling belly, soft boy-boobs, and a huge butt sticking out behind them.
The virus would be highly contagious, of course, and impossible to stop. But should it be temporary, allowing the victim to transform back after the illness passed, or should it be permanent and incurable?