In The Last of Us Season 2 Episode 4, Dina discovers that Ellie is immune after she gets bitten – and if you’ve forgotten why she can’t be infected, you’re in the right place.

Ellie’s immunity is at the heart of The Last of Us. It’s what drove the first game and Season 1: Tess’ dying wish was for Joel to take Ellie to the Fireflies on the other side of the country, hoping they’d be able to develop a cure to the Cordyceps virus from her blood.

Of course, we know how that went: the doctors could make a cure, but the procedure would have been fatal, so Joel murdered everyone and fled to Jackson with Ellie.

At the start of Season 2, it was quickly clear that Ellie hadn’t told anyone else about her immunity (except Tommy). As far as we know, nobody else is immune – so how did it happen to her?

Season 1 Episode 9 opened with a flashback to Ellie’s mother giving birth. She’s attacked by an infected, and while she manages to fight it off, she realizes she was bitten. Thankfully, she’s still able to give birth before she asks Marlene to kill her.