So there's a fungus in Brazil that really likes to grow in ants. Carpenter ants, apparently. A spore will land upon them, and mushroomy tendrils grow into the brain of the poor ant, changing its behaviour. Turning it into a zombie. Making it wander away or climb, even as the fungus continues to grow. And then, just as it's about to die, it bites. It gets a hold on some kind of vegetation and locks itself into place.
Before that happens, other ants (having evolved alongside this thing), will desperately try to push the ant as far away as possible, because they really don't want the zombie ant to lock itself into place anywhere near their colony. There's a very good reason for this. You see, the fungus is maturing. A fruiting body is growing out of the head of the ant. And then it bursts, scattering spores far and wide, ready to infect more ants. Some spores that miss and hit the ground will grow a secondary spore like a spike, sticking up so it can infect any ants that happen to pass by.
Now you'd think that anyone who's heard of this, and who's even remotely interested in zombies, would have put two and two together and designed some shambling monstrosities based on this. But strangely, no one has (not even the Half-Life headcrabs, which don't seem to use the zombie stage to reproduce as such). And this despite the scientists doing their best to alert the world by actually calling this the "zombie ant fungus", I don't see a lot of zombies that use the concept of the walking dead being part of the life cycle of a perfectly natural (if somewhat overachieving) organism. And since I needed to create some new zombies (or revenants) for my book, and wanted them to be something that was ultimately explicable to science, I went and put two and two together all by myself.
None of which explains why this is so much fun:
Crush them! Squish them! Annihilate them! Heheheheh....