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. Most great imitators are systemic in nature. Diseases sometimes referred to with this name include:

• Various cancers
• Various rheumatic conditions, including:
• Fibromyalgia
• Psoriatic arthritis
• Lupus erythematosus
• Systemic lupus erythematosus
• Sarcoidosis
• Multiple sclerosis
• Celiac disease
• Addison's Disease
• Pulmonary embolism
• Various infectious diseases, including:
• Syphilis
• Lyme disease
• Nocardiosis
• Tuberculosis
• Brucellosis
• Malaria