Jack figured he'd like to make a disease which rapidly changed people's sexes. Most people thought of biological sex as immutable even if they knew gender wasn't- that wouldn't be the case anymore!
He wished the disease would affect anyone who was in or after puberty, and once afflicted they would start to transform into the opposite sex. Any man who got the disease would have his figure soften and reshape into curves, his muscle mass reduced to give him more lithe muscles, breasts would sprout from his chest and his penis and testicles would shrink and vanish inside him, replaced by a clitoris and vaginal slit.
Any woman who caught the virus would see her figure firm up into masculine proportions, her muscles swell and thicken, her breasts would recede into masculine pecs, and her vagina would seal up and have testicles move inside it while the clitoris enlarged into a phallus.
Just thinking about the changes was getting Jack horny. And to make it even better, he added that the transformation would have an aphrodisiac effect on its subjects. Their new hormones and genitals would fill them with arousal, and they would seek other people of the sex they had previously been to fuck.
Before he wished this virus into existence, he had to decide one more thing: who would the patient zero be?