Paperback novel, read it in the seventies, I think. OK: Civilzation is falling apart, humanity divided by alien infection that leaves the infected with gray skin. Main caracter driving through the apocalyptic landscape saves young lady from the mob, then torn between care and fear as his new companion turns gray. Next: the quest to get her to a rumored safe place for the infected. There it turns out that the infection source, an artificial meteorite, is a cosmic gift from an advanced species trapped on their dying planet. The infected humans are actually not sick, but incredibly improved beings on all levels. The story ends with the main caracter (yet not infected) making a leap of faith into the room of his infectious loved one... Not bad plot recall after 40+ years, I would say. If the book doesn't exists, it should be written by someone. Would make a great movie too!!