Okay, before I can offer specific tips you need to consider:

- What kind of person is the character? Would they hide or deny that they are getting sick? Would they not notice it and fail to take care of themselves properly? Or would they notice it immediately and get an anxiety from it?

- How will characters around them react? Are they cared about by other characters? Have other characters noticed something is wrong, or are they surprised when they discover the character is sick? How do they help the character? What are they feeling upon realizing this character is ill?

- What does the character have? Is it a real illness, or a made up one? If it is real, try to do as much research on this illness as you can so you can show symptoms and buildup realistically. If it's fake, imagine you are a doctor trying to describe this illness so people can understand the symptoms of this disease, as well as the severity of it and the effects. Also make sure you're aware of if this sickness is contagious or not. The more you know about an illness, made up or not, the better you'll be able to write it.

The character is the kind of person who would try to hide it until it is necessary to acknowledge it.

The way I have it written currently the characters around them do care and are starting to get suspisious that something is wrong.

The illness is a real illness because the book is realistic fiction. The illness I chose is strep throat. I can always change it though.