The Tank. It's likely that the Tank was a very strong human before. Maybe a construction worker or a bodybuilder. The virus magnified his strength to an extreme level. He can now pick up and throw heavy objects at survivors. He has this thick, almost impenetrable skin. It's like the virus saw his physical potential and turned him into this brute force special infected to stop the survivors from escaping.
The Hunter. This special infected was probably an athletic type of person before the infection. It has great leaping ability and can quickly pounce on survivors. Its back story could be that it was someone who was outdoorsy, like a hiker or a runner. When the infection hit, the virus enhanced its physical abilities while also turning it into a mindless killer. It uses its claws and speed to take down survivors.
The back stories involve the survivors' struggle against the infected. Each survivor has their own pre - apocalypse life. For example, Coach was a high school football coach. They are all trying to make it through the zombie - infested world to find safety.
They add depth. Knowing the survivors' back stories makes you care more about them. You feel more involved in the game as you're not just playing random characters but ones with a past.
He might come from a line of forgotten magic users. His family could have had some connection to shadow - based magic that was passed down to him, even if it was a dormant power at first.
One possible theme could be survival. Since Ashley Reeves was left for dead, her story of fighting to stay alive would be a major theme. Another theme might be betrayal, if she was left in that state by someone she knew. And also, resilience could be a theme as she must have had to be strong to endure whatever happened to her.
Ghost stories may have originated from our ancestors' attempts to deal with death. When someone died, especially in a tragic or unexplained way, it was easier for people to create a ghost story around it. It was a way to come to terms with the loss and the mystery that death brings. Also, around campfires, storytellers might have told spooky tales to entertain and also to pass on warnings about certain places or behaviors that were considered dangerous.
One origin could be from people's fear of the unknown. In ancient times, when humans faced natural phenomena they couldn't understand like thunderstorms or earthquakes, they might have created monster stories to explain these events. For example, a huge, unseen force causing the ground to shake could be imagined as a giant monster underground.
Not everyone is infected. There are various factors and plot developments that determine who gets infected and when.
One origin could be a magical accident during a spellcasting competition. A mage might have tried a very difficult and untested spell, which misfired and turned them into a wild mage.
In some fanfictions, Hermione could be attacked by a new, unknown magical creature that has been unleashed by an evil force. She might be left for dead because the attacker believes she is already dead or simply doesn't care. It could also be that she is betrayed by someone she trusts, like a fellow Hogwarts student who has turned to the dark side, and they set her up to be attacked.