Masks
We see Paul, Sarah and Travis wearing masks at the beginning of the film. They are symbols that there has been an outbreak that threatens them and has the potential to be airborne.
The Red Door
The front door to the house is painted in a vibrant red. It is a symbol of danger, and that there is potential for death if you leave the home.
Muffled Screaming
We hear Will screaming with a gag in his mouth and a bag over his head. This scream is a symbol that he either has something insidious waiting to come out from within him, or that there is a human being crying out for help. Paul and Sarah must decide which it is.
Brother-in-law
Paul takes away the bottle of whiskey he is sharing with Will after Will tells him he doesn't have a brother, which doesn't match up to what he told Paul earlier. In taking the bottle away, Paul's gesture symbolizes that Paul has lost trust in Will because of this lie.
Open Door
The front door is open after Travis finds Andrew in another room during the night. It is a symbol that someone in the house has been infected and that this individual is a danger to everyone else.
