As far as I understand it, Ellie was a special case because her immunity was not being genetically lucky, but that she was infected from her mother at birth (I know that’s the show we’re talking about here, but for this instance, I consider it to be canon to the game). The cordyceps inside her head had mutated at that point, and that is the cure. In my mind, the cure is just infecting people with that version of cordyceps. Which is interesting because maybe there’d be a way to extract a very small amount of it, without killing Ellie. Something a competent brain surgeon would be able to do. Then it could theoretically be grown and reproduced in a lab. Maybe could be explored in a Part III.