This book is among my favorites as well, it is called Wither by Lauren DeStefano. Here's a little about it: Medical science grew so advanced in the 21st century that it created a generation of perfect human beings. All of the big diseases and cancers are gone and no one really knows how long this generation will live. However, the descendants of this generation are not so lucky. At the age of 25 for males and 20 for females every single person contracts an illness and dies.