Not by Fire, Nor by Ice, but by Fungus
It seems the zombies that end our world will evolve not from bacteria or viruses, but from fungi. Four newly discovered species of fungus in the Brazillian rainforest reproduce by infecting ants and taking over their brains. They force their victims to travel to an ideal location for spreading spores, then kill them and grow out of their dead bodies.
This is it. This is how it ends.