The Walking Dead: World Beyond suggests an origin for the zombie virus when flashing back to the start of the apocalypse in Sunday's episode 107, "Truth or Dare." When a scavenging Huck (Annet Mahendru) finds a Marine's EGA, it triggers her memories from a decade earlier as a military sniper alongside Drake (Gil Perez-Abraham). The pair are deployed into city service tunnels when commanding officer Wilkins (Dave MacDonald) orders their unit to attempt to contain the crisis and separate the well from the sick, telling the Marines that the "source or nature of the current crisis cannot be confirmed."
When Huck (real name Jennifer) and Drake wade into the tunnels to locate civilians and escort them to safety, with orders to eliminate any and all hostile targets, one soldier reports hearing the sickness is space-born:
"I heard it came back in a rocket. That it started in space," the Marine says. "Somebody breathed it in, it turned their stomachs, and then they got on a plane…"
He's unable to continue the rumor when the unit is attacked by hostiles, identified by their lower body temperatures. Jennifer deduces that the things that used to be people can be taken out with a single headshot.
Later, Jennifer and Drake's unit receives "Sunset protocols" from central command. Under this protocol, all units in the hot zone are directed to kill all targets in their grid. When Jennifer protests, Wilkins makes their orders clear: "You put down anything still walking. Friendly or unfriendly. Civilian or hostile. Living or dead."
Huck's series of flashbacks tie into The Walking Dead, where Lori (Sarah Wayne Callies) and Carl Grimes (Chandler Riggs) witness the military's bombing of Atlanta towards the start of the apocalypse. In the first season of Fear the Walking Dead, the National Guard enacts a safe zone in the California neighborhood home to the Clark family as the military carries out Operation Cobalt.
Operation Cobalt, a failed attempt to contain the spread of the still-undefined zombie virus that causes the freshly dead to reanimate as flesh-hungry "walkers" or "empties," ended with the bombing of infected zones and kill orders inside impacted areas.
The cause of the global outbreak remains unknown as the virus, which infected every person on the planet, has never been fully defined by franchise creator Robert Kirkman.
Asked to reveal the origins of the zombie plague on Twitter earlier this year, Kirkman replied: "Space spore." The creator later clarified his response was a joke, adding that he would "never reveal something like this in a tweet."
When pitching his and artist Tony Moore's comic book to Image Comics, Kirkman falsely claimed The Walking Dead's zombie apocalypse was created by extraterrestrials that used zombies to weaken the world's infrastructure and ready it for invasion. Kirkman admits it was a lie to sell executives on the book.
World Beyond involves the Civic Republic Military, or CRM, a shadowy organization supposedly seeking a cure to the zombie virus with scientist Leopold Bennett (Joe Holt) working out of a CR research facility located somewhere in New York. A series of post-credits scenes reveal the Civic Republic is conducting zombie experiments under Dr. Lyla Bellshaw (Natalie Gold).1comments
"Maybe years after it's all over I'll just casually mention it in an interview. That seems like a very J.K. Rowling thing to do," Kirkman said about the virus origins during a 2018 Q&A. "It couldn't be less important to the story and the lives of these characters. It would be completely out of place in the story. Honestly, if a scientist from Washington came to the characters and told them what happened ... it wouldn't change their lives at all."