Factual error: A doctor is seen using a syringe to inoculate the individual with the disease. It is clear that the containment gloves and sleeves he is using are not proper as you can see his skin between the gloves and the sleeve. If this were a real containment unit his entire skin would be covered. (00:31:00)
Factual error: With a significant portion of Marcy's legs either shaved off her body or in a state of severe rot, it's doubtful she would even be able to stand up properly, let alone run away from a mad dog. And if she'd already lost as much blood as we later see smeared around the bathroom fixtures, she probably wouldn't even be conscious. (01:10:45 - 01:37:50)
Answer: They don't know about it. The only resident in town to contract the disease was the hermit. The sheriff even remarks to Deputy Winston on his radio that the kids at the cabin were going on a killing spree and that they had some kind of skin disease, indicating the disease was heretofore unknown to them. It's possible that the hermit's dog was the carrier.
Phaneron ★