Calling it a virus is a misnomer. The scientists were developing nanotechnology. One of the dangers of nanotechnology is that nanites need to to be programmed to know when to shut down and programmed to limit themselves. If such programming is not included then nanites have no limits and they become like a cancer, sweeping through everything.
That's what the accident was. The programmers programmed the nanites to partially limit themselves but someone forgot to program a shut down into them. So the very first human test subject left the lab with active nanites in his bloodstream infecting any male he sneezed on. (The nanites were programmed to limit themselves to only changing males into females, not females into males.) This was the first day.
*****
On the evening of the second day, Jake was having sex with his girlfriend Tanya. The sensations were intense as Jake's penis seemed to get more sensitive. Both of them were lost in the sensation and Jake had the most intense orgasm of his life. He shut his eyes and felt deliriously happy. But he noticed he also felt like he had just cum every drop of semen in his balls inside her.
When he opened his eyes, he noticed his cock seemed to not only be getting flaccid but receding away, and his nipples were weirdly erect. He breathed heavily and felt his chest starting to expand, soft breasts forming. Something seemed to form deep inside his crotch and it tugged his testicles up inside him and flattened his scrotum. A slit formed between his legs (or rather her legs) and his penis formed her tiny sensitive clitoris.
Tanya heard his heavy breathing and had watched her boyfriend transform in astonishment. When Jade's eyes met hers they were both in disbelief and for a moment didn't do a thing.