After a series of nightmarish attacks by, of all things, common dust mites, scientists begin to see horrifying ecological changes rapidly occur throughout the world.
Species begin dying off at unprecedented speed.
The algae in the oceans disappear. Food chains begin to collapse. Outbreaks of famine, disease and war begin to break out, first in third world nations, then spreading to engulf every corner of the world. It's happened many times throughout the planet's four billion year lifespan, once 65 million years ago when the dinosaurs roamed the planet. Go read the book.