In 1953 a certain laboratory on an undisclosed location devolved a serum that could genetically modify humans, giving them enhance speed, agility, strength, and brainpower.
Scientists found a way to modify the serum so that it could only activate itself in the presence of a Y chromosome, thus isolating the effects to men.
The serum was a success, and sales skyrocketed just a few weeks after it’s release.
What the developers did not anticipate, though, was the human body’s incapacity to handle the serum.
The mental and physical over-exhaustion triggers a mental decay which starts out slow, but speeds up within a few months after usage of the serum.
The brains of the users are left with only the most basic survival reflexes, transforming the users into strong, fast, agile, emotionless human shells, devouring any life form in their path.
Bites from the affected individuals could place copies of the rouge serum into the bodies of the bitten, giving them the symptoms.
Shortly after, the serum evolved into a sort of genetic virus, causing mental decay in just days. No one was safe.
…except the women.