Invisible Illnesses: An A - Z of Hidden Conditions and Rare Diseases.Millions of people in the world are living with hidden illnesses, many with no visible symptoms at all. Due to their unseen nature, and some of them sharing similar or even the same symptoms, many of the disorders are hard to identify and can often go undiagnosed or misdiagnosed for months or years.This book is designed to be a brief snapshot into some of the most unknown and misunderstood conditions.