In this fanfiction, it's possible that Max's sickness could be some sort of super - villain - created illness. Maybe a villain has found a way to target supers specifically through diseases. This would lead to a race against time for Max and his family to find a cure before his powers are permanently damaged or he gets too sick to fight back. Also, his friends at school might start to suspect something strange is going on when he misses a lot of school because of his illness.
Well, in the fanfiction, his family would surely be worried about him. His sister Phoebe might try to use her powers to heal him, but it could backfire as they don't fully understand how their powers interact with illness. And his parents would be hovering over him, trying to make sure he gets better while also keeping his super identity a secret from the outside world.
Maybe his superpowers go haywire while he's sick. Since Max is a super in The Thundermans, getting sick might have unexpected effects on his abilities.