Another possibility is that one of the other characters in PLL has a hidden motive. For instance, someone who is jealous of Allison's popularity or someone who she had an argument with in the past. They might have found a way to make her sick without being suspected at first.
It depends on the context and development of the story. Sometimes love alone can overcome many challenges, but in other cases, external factors might play a role.
The Blair Witch is not a real entity. The 'Blair Witch' story was a fictional creation for a horror movie. However, it was presented in a found - footage style which made it seem more real. The movie creators used a lot of elements like the mystery of the Blair Witch legend, the idea of a haunted forest, and the disappearance of people to build up a spooky atmosphere. This was all part of the marketing and storytelling to scare the audience.
Maybe she would begin with a significant event that set the tone for the rest of her story. Like, 'It all started on that stormy night when I made a decision that would change my life forever.'