Not sure when exactly I read this, may have been in the 90s or early 00s.
The novel is set in north America I think. Some kind of energy beams or rays from space rain down on Earth. Where they happen to hit a human, that person gains some kind of power. From what I remember the powers were not superhero-like, they may have been related to whatever the person was doing at the moment of being struck. There was a woman who was hit whilst.. erm, "in flagrante delicto" and gained a seductive power over men. There was an antagonist who was an old man that dies just as he is struck by the energy, he becomes a kind of undead and starts killing off others who have gained powers (including a scene where he kills a messiah-like character who is speaking to a crowd).
I can't remember if the book explained the origin of the space energy, I think the climax involved the remaining people with abilities facing off with undead guy in a forested area.