The infected children were doubled by dwarf stunt people.
The photo featured in the beginning inside Clint's (Elijah Wood) bedroom is an actual picture of Wood as a kid.
The second time that Elijah Wood plays a character who fights monsters in a school. The first time was in The Faculty (1998).
At one point, Wade ([Rainn Wilson]) insinuates that Clint (Elijah Wood) sneaks around like a Hobbit. Wood played the Hobbit Frodo Baggins in the "Lord of the Rings" trilogy.