The Last of Us episode 1 kicked the series off with a pretty shocking virus outbreak scene, but a new theory is proposing what caused the outbreak, and it’s a big departure from the games.

Although it’s known that the zombie virus has something to do with fungi, which was confirmed in episode 1’s opening scene, the show still hasn’t confirmed what suddenly kicked off the virus’ spread in 2003. However, The Last of Us episode 1 touches on several seemingly inconspicuous details that, when combined, paint a pretty clear picture of what caused the outbreak.

Based on the video game of the same name, The Last of Us follows Joel and Ellie as they attempt to travel across the United States in order to get Ellie to a group that can help cure the virus. In the TV series, the fungal virus outbreak occurred in September 2003, with it still going strong 20 years later.

While an in-universe interview from the 1960s did reveal that the fungal virus had something to do with climate change, the method through which it spread so suddenly is still unknown. The Last of Us episode one has revealed all of the pieces required to figure out this puzzle, and the virus mystery may have finally been solved.