Okay. Some thoughts. *SPOILERS*
We know that the little boy Paulie is sick immediately on arrival to his aunt and uncle's home. His cousins Nicky and Leah (who are younger than Miranda and Casey) get infected next and Miranda seems to be fighting it, but also succumbs to the infection. Later at the end we see that Casey also seems to have contracted what ever the disease is. So it starts with the very young and there could be a chance that it will infect the adults also, but take longer.
It also seems that the children are infected by an alien host or parasite. You get the impression that the bodies of the children are dead because they are not bothered by the cold and also you see on little Leah just before she kills her mother that she has some odd mark on her skin that looks like it is dead skin. The ambulance and police do not arrive, we see the radio not broadcasting music, news or anything, so you get the impression that the infection has spread elsewhere. And, of course all the kids surrounding the car at the end.
First off I had no remorse for the adults being killed in this movie. The uncle was creepy on how he acted around Casey. Casey's step-father Jonah was a crappy father who beat his son for breaking up his business talk with the uncle and for all he knew it was an accident (of course we know it wasn't, but he didn't) and then he spoiled his 'princess' daughter Miranda who attacked her aunt in the face and was screaming non-stop. None of the adults paid any attention to the kids when they said they were sick. So goodbye adults. Except Elaine (Casey's, Miranda's and Paulie's mom) who was more than likely going to die since Casey is infected at the end, but you feel some remorse about her.....although she did not go up stairs when Miranda was going absolutely nuts and screaming like a banshee. I don't know. But, if my child was hysterical like that I would be with her. So crappy parents.
The kids were better actors than the adults in this movie, but then again their main acting was to be creepy and most kids can do that without trying in real life. The dialogue was poorly scripted as well as the editing. Could have been a lot better, but I will give it a 'C' for trying.