Villainous Quirk, Heroic soul

Prologue

It was a cool winter day in japan when the fire nation attacked. No. Wait. Wrong amazing show. In this one we find our main character riding a train on his way to what would be the day his future at a hero would be decided. The UA university entrance exams. The most prestigious hero University in all japan, heck maybe even the whole world.

Here on this train we find Izuku Aizawa browsing through his phone, trying to calm his nerves. Unfortunately there wasn't any recent hero news that he didn't already know about, so he was reading a list of top 10 villain schemes that changed the world in an ironic way. From a villain trying to melt the polar ice caps to flood the world, but fixing the hole in the ozone instead. To a villain trying to take over the United States, but instead getting a new and better judicial system in place.

Though his favorite one was number 3, a woman who is taught in high school in both Quirk ethics and History. The villain was named Plague. She was an American, and one of the first people in the world to develop a quirk. Her quirk allowed her to develop any disease she could desire, as long as she knew how it worked. Though because of quirk discrimination when they first started appearing, she hid the fact she had one quite well. So no one batted an eye when she started to study to become a microbiologist.

She was a very patient and intelligent women, she studied for years to make sure she could create the perfect disease so it would never be cured. Her goal? The eradication of man from Humanity. She believed men to be nothing but a plague themselves, and thus should be eradicated. She was damn clever on her application as well. Due to her intelligence, she gained prestige across the whole world in her field. She found cures to many diseases, including cancer and AID's.

No one knew that while she was traveling she was purposefully spreading 2 plagues. The first one was a cover, to make sure that if she was caught, no one would suspect that she also created and spread her main virus. This one was simple, and easily cured by other microbiologists, though it was in fact deadly, she was more focused on it be contagious.

The other one though, was a virus that latched on with the first and only effected men. The

Problem was, she was very patient. She knew for her goal to succeed, she wouldn't be able to live long enough to see it happen. She knew that if she made her quirk work to fast, it would be quickly discovered. While she made the quirk highly resistant to curing, it was impossible to make one incurable. It was also impossible to make it that no man could be resistant to it. Though as time would go on, it would do exactly as she wanted it to do. Which was once it infects a man, it made it difficult to produce sperm with the Y chromosome and eventually make it impossible, thus only able to produce daughters.

Eventually the early hero's discovered that the first plague would always start spreading in any country after she appeared. Since she didn't want her quirk to be discovered, she went in peacefully. What she didn't know was that doctors had discovered a way to tell if someone had a quirk or not, so she was forced to reveal what it did. Without a way to suppress quirks at the time, she was deemed too dangerous to let live, and was sentenced to an immediate death.

She didn't care though. She finished with her task.

After giving them the information on how to cure the obvious plague, so as she planned, they didn't suspect a second one. Everyone found it creepy how she smiled in the electric chair.

So as everyone's lives continued on as normal, it was quickly discovered how the male birth rate was declining. People first suspected it to be another oddity associated with the appearance of quirks, so nothing was done about it. That is until after 150 years since the day quirks appeared, male birthrate didn't go up. In fact now only 15% of humanity was male, with each generation having less and less.