The Great Imitator (also the Great Masquerader) is a phrase used for medical conditions that feature nonspecific symptoms and may be confused with a number of other diseases. The term connotes especially difficult differential diagnosis (DDx), especial potential for misdiagnosis, and the protean nature of some diseases. Most great imitators are systemic in nature or have systemic sequelae, and an aspect of nonspecific symptoms is logically almost always involved. In some cases, an assumption that a particular sign or symptom, or a particular pattern of several thereof, is pathognomonic turns out to be false, as the reality is that it is only nearly so.