Threshold of Midnight (en Inglés) - Jeanne Donnelly

Diana is plotting her divorce when she's accidentally locked in an underground shelter. While trapped, a green fog covers the planet. The world ends not with a bang but with a whisper as people turn to dust. When Diana finally manages to escape, life as she knew it is over. Her husband, friends, and everyone in town have disappeared, and there are piles of a strange, yellow dust in stores and houses.

Heading out to find survivors, Diana comes upon Gracie, a small child who thinks the Virgin Mary is talking to her. Diana believes the child is traumatized as she delivers messages from the Virgin Mary, but when the things Gracie tells her start to come true, it's time to decide if this child is a prophet, delusional, or humanity's last chance.

Along the way, Diana will meet two men, one will save her, and the other is hunting the child.

Sometimes you don't know how strong you can be, until you're protecting a child who may be humanity's last chance.