As fans of The Walking Dead continue to walk the show’s spin-off series, some wonder how the virus began. Here’s what you need to know.

When it comes to The Walking Dead, there are two things fans have come to expect from the brand: zombies (or walkers, as they call them) and a lot of walker-related deaths. Well, that’s the apocalypse for you!

Since The Walking Dead debuted in 2010, fans have enjoyed watching Rick Grimes and his crew battle their way through dangerous situations as they try to survive the zombie takeover.

However, as The Walking Dead has continued expanding its long list of spin-off shows, a lot of fans have wondered where and how the original walker virus started. So, here’s everything we know.

Walking Dead kept virus origin a mystery until spin-off

The Walking Dead virus’ origin was explained during a post-credit scene in The Walking Dead: World Beyond, where it was revealed that the virus was released from a biomedical facility.

Walking Dead creator Robert Kirkman deliberately chose not to reveal the comics’s zombie virus origin and, though the show came close to explaining their existence in the first season, they soon backed off and left the mystery alone.

However, one of TWD’s spin-off shows — The Walking Dead: World Beyond — seemed to have finally lifted the curtain on the mystery through this post-credits scene.

The scene takes place at a biomedical facility in France where one of the former researchers has returned in hopes of continuing her work to discover a cure for the walker outbreak, but she’s confronted by an unidentified survivor.

The survivor reveals that the facility housed numerous teams (the “Violet and Primrose team”) who worked on a project that ultimately became the walker outbreak virus. Since Primrose team traveled to the U.S. shortly before the outbreak, the survivor believes that other countries were involved in the study too.

However, though the survivor blames the researcher and teams for creating the virus, the researcher turns the accusations around as she states that: “When [the outbreak] happened… When you did what you did.”

She’s seemingly implying that, upon learning that the walker virus was being developed, the survivor’s group must’ve attacked the facility to stop it and accidentally released it, making both parties to blame for the world’s state. If that’s true, and the destruction of humanity as we know it was the result of an accident, then that’s a very harsh pill to swallow.

Though Kirkman believed The Walking Dead would be a better comic and show if the origin of the virus was never revealed, this scene definitely gave closure to the fans who didn’t want to be strung along about the end game.