Is the end... (SPOILERS)

... a dream?

It seems established as a rule that every time Travis is "dreaming" the movie's aspect ratio changes slightly. In fact, the change is not sudden, the image's height slowly decreases when entering the dream, and slowly increases back when exiting that same dream. It's very subtle even, due to the fact that most dream sequences are very dark and it's hard to distinguish the top and bottom edges of the image moving during the transitions.

At around 1:17:30, as soon as Will and his family leave the room holding Paul at gunpoint, the longest "dreaming transition" of the movie starts, and the picture slowly shrinks for like four minutes until it reaches the "dreaming aspect ratio" exactly when Paul shoots Sarah, and it then stays the same throughout the end of the movie.

To be honest, the movie doesn't make much sense. It's a complete mess from both the narrative and survivalist points of view. Any answer could be considered correct for every question left unanswered, and there are a lot of dumb decisions made from a survival point of view. And that last detail, the whole thing going into "dream mode" during the end doesn't help at all. Because if the change in the aspect ratio every time Travis had a nightmare was done on purpose, then it surely has to have a meaning when it happens for the whole ending, hasn't it?

Who brought the infection to the house? Any guess is good. For all we know, Stanley, grandpa's dog, could have been the one bringing it into the house in the first place, infecting his owner (Bud), then Travis (he took charge of the dog and even let him sleep in the same room) and subsequently everyone else. They were careful washing up, wearing gas masks and gloves, yet they let the dog roam freely outside and inside the house. We are never given any details on the disease, everyone might be already infected and the "24 hours quarantine" rule something that was just thought of but not really effective, the disease's behavior changing from person to person. The whole family could have been already infected when the grandad showed symptoms, ironically dooming Will's family when they took them as guests.

But to stay on topic, the question is puzzling: why on earth would the director change to a "dreaming sequence" picture format for the whole ten last minutes of the movie?