Other mistake: When the deputy pulls his gun on the sheriff, he takes the rifle and hangs it round his neck. In the next shot he's doing it again. (01:07:35)
Other mistake: In the scene where Judy and Becca are strapped to beds in the high school and the principal is coming with a pitch fork, at first there is a close up of the fork (when the principal is dragging it) and its dry with no blood, the principal walks over to his first victim and right before he raises the fork it is shown covered in blood so much that some has ran on to the floor.
Answer: Possibly because the Government didn't want word about the virus getting out to the general public so they ordered the solders to shoot the non infected people in order to silence them all, and then created a cover story about a chemical plant exploding in the town.
Trainman