Harvey had always been in and out of the hospital. It was something he'd been familiar with since he was a kid. He was prone to getting sick in the overbearingly hot atmosphere of Arizona. His mom was always saying how much it costs to keep him in the hospital until he got fed up with it. So he demanded to move to Forks with his father, his sister Bella getting sent with him. He enjoys the cold of Forks, it was better than having a 103-degree fever and it being 100 degrees outside and only slightly colder in the house. So he thoroughly enjoyed his new group of friends' usual cold skin.
This will not be following the plot but I'll try to add as much from it as possible. The reasoning behind this is I've only watched the movies with my dad and he always skips the 2nd movie so I have no clue what goes on in that one and I don't feel like sticking to the original plot cuz I'm lazy.