Devilman Lady/WMG

The Devil-Beast Syndrome is related to the X-Gene.
A disease dormant for years? Check. Activated as a teen? Check.

Jason Bates is responsible for the Devil-Beast Syndrome.
As a human, he is a Complete Monster Mad Scientist working for some evil company that makes viruses (Gentek or Umbrella) and is obsessed with the paranormal (like Arkham in Devil May Cry). He has found the disease from the past and turned it into a much more deadly virus with demonic power. When he is pursued by the guards who defends the company, Bates fled with the sample of it and unleashed it to the world before being shot to death by the security.

If he is Killed Off for Real, then the Jason Bates that Jun will meet is the Devil-Beast Syndrome who thinks as him.

Jun has been in a Dying Dream all along.
Therefore, the entire world is apocalyptic just like in Devilman.