Jeff turned himself back to his original body. Then he entered the settings to create the virus.
It would be contagious for a week prior to the symptoms setting in, so there would be no way to quarantine those who had it. Those who caught it would have the first week of contagion with no symptoms, then a week of gradual weight gain and unusual hunger, then the transformation would spontaneously happen.
He went back to school in his normal body, elated to share this with everyone.
After a week of exposing his family and classmates to the virus, his symptoms set in. His tummy started getting softer, his body hungrier, and he had to eat bigger lunches and dinners to compensate. People didn't notice the first few days, but soon people started to ask if he was okay. "I feel sick," he told them. "Maybe I should take a few days off."
"You're fine, Jeff, get over it." was the general reply.
Jeff timed it so the transformation would happen in the middle of one of his classes.
"I feel weird," he moaned out loud, and all the heads turned to watch him. "My body is..." His belly suddenly blew up, getting heavier and heavier. His butt got bigger and softer, filling and then straining his trousers. He was forced to stand up, unbuckle his belt, and pull his trousers off in front of the whole class, because the strain of his widening hips was too painful to bear. "What's happening?" he acted, knowing full well this would soon happen to everyone else in class. "I'm so short and fat! This is horrible! My face!" he ran his hand over his forehead, bulbous nose, thick lips, wideset jaw, jiggling breasts. He ripped his shirt off, as it was barely containing his overhanging belly. He jiggled the belly in feigned horror.
He continued to yell in terror, wearing just his boxers and socks, as the transformation continued and soon people noticed something: "Whoa, Jeff, you look like Danny!"
All eyes turned to Danny who was beet-red from humiliation. Jeff didn't look exactly like him yet, but it was getting closer and closer. "Shut up, he does not!" Danny muttered.
But the short fat kid in his underwear fondling his belly and moaning in horror was more and more identical to Danny.
"Alright, everyone clear out, we need to get Jeff to a doctor," the teacher finally said. No way a doctor could successfully treat an inexplicable full-body transformation like this, but we can't all sit here and watch.