In the world of The Last of Us, people can become infected primarily by bites and cuts from other infected. In Episode 5, the showrunners finally introduced spores to the playing field, making it a lot easier for people to become infected. Originally, showrunner and creator Neil Druckmann said there had to be a “dramatic reason” for spores to make their way to the show, and with the episode’s climactic end, we have our reason.

This story contains spoilers for The Last of Us Season 2.

As we saw in the trailer last week, fans knew that spores were coming to the show, and now we know how they’re created. At the end of the episode, Ellie (Bella Ramsey) chases Nora (Tati Gabrielle) through a hospital basement where spores have been running rampant for years. Nora thinks Ellie is going to become infected as well, but the revelation of her immunity is quickly realized.

“When Neil and I were laying out Season 1, one of the questions we had was how to deal with the idea of spores. That underground, in certain spots, there was a discovery that spores were in the air,” explained showrunner Craig Mazin, during the behind-the-scenes featurette. “They hadn’t gotten out. Let’s just seal that off and pretend like it’s not there. Denial. It’s our favorite.”

The set created for the show mirrors the game’s elements almost perfectly: the thick fungal growths with the earthy colors, and of course, the infected victims embedded into the twisted columns.

“What we’re seeing are people who have been alive for years, decades,” Mazin added. This is where the dramatic elements come into play.

While looking for her son, Elise (Hettiene Park) discovers that her son has been infected, as well as several people below, because the infection is in the air. She seals in her son, along with the others, sacrificing him for the greater good.

“We come back to similar choices that characters have made on this show. ‘Do I save the person that’s closest to me that I love the most, or do I make the ultimate sacrifice of my own child to save the community?'” explained Druckmann. “Here we got to see someone who made the opposite choice that Joel would have made.”

Mazin continued by explaining how these infected work: “They exist in this endless tortured state where they breathe air in, and they breathe spores out. If anybody else was down there, they, like Nora, would be infected. But Ellie is not like anyone else.”

Spores were originally used in the game with Ellie’s reveal to Dina that she’s immune. That was changed to Ellie taking a bite for Dina in Episode 4 during the subway chase scene.

New episodes of The Last of Us air Sundays at 6 PM PT / 9 PM ET on HBO and Max.