This movie was created as a proof of concept for a feature film of the same name.
Every cast member did the stunts themselves. Kassie, who plays the main character, did nearly 50 dirt bike jumps of which only 5 were actually filmed. She hit her marks so methodically that after 5 takes the directors knew they had more than enough.
All of the effects are practical with the exception of light removals, safety equipment removals and two shots of CGI smoke.
The breaths of the child played by Shannon Caruso are in fact the director Alex Lee Williams with his pitch turned way up. He also plays all of the sounds of the zombies with the pitch turned way down.
The directors were frustrated that nobody would fund an action film from filmmakers that had never shot action before, so they put up their own money with their producers to prove they knew how to do it.