Inspired by a drawing I did a while back about what I thought my 'Personal Demon' would look like.
An asteroid hit the planet.
No-one bar some dedicated meteorologists noticed, because by the time it hit the surface it was the size of a marble. Sadly, the alien disease that had survived the depths of space on that asteroid survived the atmospheric entry.
The disease was Psionic in nature, chewing up one specific emotion and leaving the others completely untouched.
A world without fear is a dangerous world. There were no longer any voices in the back of people's mind that would warn them that it might hurt if they threw themselves out of a window. Nothing to say 'You cant fly a plane' or 'You shouldn't build bombs to prove a point'.
Some people survived the disease, though. As the disease attempted to consume their fear, their minds reacted violently to combat the disease immediately - their smallest fears suddenly bloated and exaggerated in their psionic awakening.
As their minds changed, so too were their bodies now capable of changing. They could allow the fears that plagued them constantly to consume THEM, letting their own personal demons out of their minds and into their bodies, shifting flesh and making them creatures of fear made manifest.
Now, the Bogeymen are humanity's white blood cells - being near a bogeyman... even one who looks like a human... allows humans to feel fear again as the presence of the Bogeyman fights the disease in their minds. The majority of this fear is aimed at the Bogeymen themselves, but its a small price to pay until someone can work out how to eradicate the disease once and for all.
... So yeah. My head can be a weird place sometimes. *Finger Guns*
Basically were-fears fight against alien disease that can probably make its own twisted versions of the Bogeymen that are exemplars of other emotions.