The disease, or whatever it turned out to be, was highly specific in targeting anyone with XX chromosomes. Within a year, every woman, every little girl, all of them had died. The XY chromosomes didn’t get a free pass either. Over 75% of them…of us died as well. .
It was that cloud the Earth passed through. It defied chemical and radiological analysis, so at first we didn’t realize how devastatingly fatal it was. It had drifted into our solar system, and our planet had the misfortune of being in the portion of our orbit with which it intersected. The first deaths occurred only weeks after contact.
A world population of over eight billion reduced to just less than a billion, all male. The last time the Earth held just a billion people was at the beginning of the 19th century.
But most people probably thought that moot since without women, the human race couldn’t reproduce and repopulate. The little boys born before their mothers died would be the last generation. That’s what most people thought.