Wow! What a lovely book! 
 I though I would cry my heart out like I did reading The Fault in Our Stars but how completely different this book is! 
 Madeline is sick and has always been. She suffers from SCID which basically means that she's allergic to everything. One day, new neighbours moves in with their son Olly. Olly and Maddy start discussing online and they fall in love. Madeline then starts to question the status quo and all that she ever lived and was told. She's in want for more : travel, eat all the food she want's, meet with new people, etc. 
 (view spoiler)[ One day, she decides to run away with Olly to Hawaii. After a day there, she falls sick and almost dies. While she was there, the doctors ran some tests and figured she never had SCID. At this point, the whole story of Madeline's life falls apart. (hide spoiler)] 
 The book is a reminder to never take what we have for granted. To question all that we know so that we don't end up trapped somewhere we don't want to be. To enjoy all that surrounds us and find beauty in the most tiny things. I belive it can also be a analogy to all the people who suffer from disease that have them trapped inside a malfunctionning body with a fully fonctionnal brain.