uh oh, The Earth was under siege by showers of meteors, but it was more than that, the meteors carrying a creature, a parasitic extremophile, a small bug like larva that when infects a human it will merge with the spine and mutated the spine into a giant Centipede like creature and change a person into a wandering headless zombie, they hide inside and have complete control of the body.

Since the head is missing they use their long antennae trough the new orifice to see and detect human scent. they can crawl out or retract for protection.