If the grandfather died because he had virus, there's a good chance all of the remaining family members were infected because they lived in the same house. The premise that you can only be infected by touch is ridiculous... if that was the case then everyone is infected because everyone touched one or more times during movie. Remember when families met? Everyone shook hands.
The writers were purposely depicting Travis as a disturbed young man throughout the movie. I think this was intentionally done so all unanswered questions the audience had would lead to Travis. Door being unlocked could not have been done by the child. He's too short. The movie does make a lot of sense.