Once upon a time...
Ah, who cares about that?
Long story short, one day, someone somewhere noticed that their body was changing. Not in any usual way, but their chest was expanding more than a man's should. Or maybe they were growing more facial hair than a woman should.
Not knowing that this was a disease, this person went about their daily business without a care in the world. Pretty soon, many more people were infected with this strange disease, so many that the CDC was soon called in to investigate.
Because of the gender-changing effects of this virus, the government officials named it The TG Flu (TG for transgender). No one was immune. Symptoms manifest in time, a little at a time, eventually leading to a complete reversal of the infected person's biological sex.
The disease is not dangerous, but cases have/will spread throughout the world. We can follow Patient Zero, or someone who's been infected during the early days. We can even see what the world is like some time after the disease, if we want.
And now, off we go...