Infelizmente, não há resposta verdadeira ou real para essa pergunta. O diretor propositalmente deixou detalhes vagos e questões não respondidas em um esforço para manter o tema "o medo do desconhecido" em andamento. Então, realmente, está aberto à interpretação dos telespectadores se Travis foi quem adoeceu de Andrew ou vice-versa.
"It was always important to me to never be a step ahead of the characters. We’re never going to find something out unless they find it out. I know what happened before the first frame, and how they got to that moment, all the way to what happens later, and certain things that aren’t explained. But the exciting thing to me is treating storytelling through the characters’ point of view, and really living it. And that’s going to frustrate a lot of people."
Veja outro trecho de uma entrevista com o diretor Trey Edward Shults:
"The storytelling in the film is very deliberate, and what’s in there is intentional, and what’s not in there is intentional. I didn’t make a movie to frustrate you! I hope you can just be in the movie and let loose, and instead of it being a frustrating thing, it’s an interesting thing. And if you dig the movie, it’s something that you can return to and see new things, or analyze, or whatever you want to do."
[EDITAR] E nessa mesma entrevista, o diretor / escritor também explica o final originalmente pretendido (e mais longo):
So there’s a whole other version of the ending that I have. It’s a final nightmare, basically, and these nightmares throughout are supposed to put you closer to what is going on inside Travis’ head and what he’s battling in his subconscious and get you closer to him, beyond just being scary or whatever. And there was a final one that was basically Travis’ reality has become a nightmare, so it’s all just a nightmare, and it was like a sort of dying fever dream where he confronts everything and it’s like his personal hell. And his hell is his house and his parents and facing what they’ve done.
Intellectually, it made a lot of sense, but in practice it was just pummeling and pummeling, and people weren’t with it emotionally. I found less is more, and I found a way to get across what I wanted to get across without beating people into submission.
Isso parece sugerir - e a entrevista mostra muito - que a doença é real para Travis no final, de onde veio. E isso sugere que há um tipo de borrão entre o pesadelo e a realidade que vem da desconfiança, do medo e da maldade.