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 female he sneezed on. (The nanites were programmed to limit themselves to only changing females into males, not males into females.) 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 Tanya's vagina seemed to get tighter. 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. When he opened his eyes, he saw a man where Tanya had been and he felt a cock pushing against his cock.