It's based on a zombie movie I watched in middle school called Bunks. 

It's about two boys called the O' Rileys who were punished for destroying their parents' garage with a rocket and were originally supposed to be sent to a boot camp....but after seeing a different bus that leads to a camp by the lake, they decide to switch their names with two people named Sanjay and Delroy and get on the camp bus while the two boys went to the boot camp. 

They went by those names and plan to stay at that camp for the entire summer. 

When they got to the camp, they meet a group of boys and gave them nicknames and doesn't bother to learn their real names. 

Things were going smoothly until a different camp group called the Badgers were being d**ks to the Chipmunks (the team the O' Rileys are on) which caused them to steal the Nameless Cup (trophy) and hide it in an old abandoned shack....they also found a backpack that has an old teddy bear, cake snacks from the seventies and an old book. 

They plan to read it by the campfire with the camp counselors when there was a strict rule on it....they read a story about a man named Anson and a group of friends that were in the lake but Anson drowned and the friends take him to a scientist to bring him back. 

The scientist brings him back....as a zombie and infected his friends. 

They were able to manufacture a cure for zombification but it doesn't work on Anson because he's been dead for too long. 

The story scared the counselors but....little did they know....it comes true. 

The full movie is on YouTube but who knows how long it'll be on until it gets taken down for copyright. 

So you'd fully understand. 

https://youtu.be/Ua8iInSIGgw