Mental illness affects a significant swath of the population and fortunately much of it is treatable. Here is a list of strange and interesting mental disorders that you might not know about.
1. The Capgras Delusion
The Capgras Delusion is a syndrome whereby an individual comes to believe that friends, family and others close to them have been replaced by imposters or lookalikes. The condition is named after French psychiatrist Joseph Capgras (Born: August 23, 1873, France) who first described it in a study published in 1923. It afflicts women more than men by a ratio of 3:2 and is often found in paranoid schizophrenics and brain injury sufferers. As you can imagine, this disorder can be devastating for loved ones who find themselves perceived as strangers in the eyes of the Capgras sufferer. Unfortunately, violent behavior and murder sometimes accompany Capgras delusion such as in the two cases of matricide documented in this clinical journal.
2. Fregoli Syndrome
Fregoli syndrome is another paranoid disorder similar to the aforementioned Capgras delusion. The difference here is that where Capgras describes a belief that people familiar to the Capgras sufferer have been replaced by fakes and doubles, Fregoli syndrome describes a belief that a shapeshifting adversary is masquerading as different people. They believe that multiple people are actually alternate versions of the same person. People with Fregoli syndrome typically feel persecuted and threatened by their protean stalker. This disorder is associated with Injury to the right frontal and left temporo-parietal areas of the brain and is believed by psychiatrists to be related to a breakdown in normal face perception.
3. The Cotard Delusion
Cotard syndrome is a rare condition first described by neurologist, Dr. Jules Cotard in 1882. This delusion is characterized by one’s belief that they are dead, do not exist or that their organs, blood and body parts are missing. People with this condition often believe that their body is rotting and they withdraw from others while neglecting personal hygiene and food consumption. Cotard Delusion is often accompanied by mood disorders such as depression and effective treatments for it include the use of antidepressant, antipsychotic, and mood stabilizing drugs and also electroconvulsive therapy.
4. Diogenes Syndrome
Diogenes Syndrome is a behavioral disorder characterized by excessive hoarding, living in unsanitary conditions, isolation and extreme self neglect with regards to personal hygiene. It is a condition that is often found in the elderly but it can afflict people of all ages, genders and socio-economic standings. It is most common among people with above average intelligence, who are over 60 years, and who live alone. People with this syndrome often develop a skin condition called dermatitis passivata, where a scaly crust develops over the skin. This is usually due to a lack of regular bathing. It is named after Diogenes, a 4th Century Greek philosopher who lived in a barrel and famously spoke to Alexander the Great with apathetic irreverence. 
5. Alien Hand Syndrome
AHS is a bizarre condition where one’s hand is subject to involuntary behaviors that make it appear as though it has a life of it’s own. It can also affect other limbs and patients who suffer it have reported being spontaneously assaulted by their demonic hand including being choked, slapped and punched. Brain scans show that AHS may stem from lesions in the brain where motor control, planning, and sensory relay, are regulated. Such lesions may occur after stroke, through neurodegenerative illnesses such as Alzheimer’s disease, brain tumors, and seizures. Certain types of brain surgery have also been reported to give rise to AHS. 
6. Ekbom’s Syndrome
Also known as delusional parasitosis, Ekbom’s Syndrome is a delusional belief that one’s body is infested with parasites, bugs and other creepy crawly things. This is type of paranoid hallucination is observed more commonly in women, especially those past the age of 40. Sufferers have been known to inflict harm to themselves in an attempt to rid themselves of the imagined parasites. The delusion can sometimes transmit to others in what is called Folie à deux, which is a term for shared madness.
7. Sleeping Beauty Syndrome
Also called Kleine–Levin syndrome (KLS), Sleeping Beauty Syndrome is a neurological condition that causes patients to sleep for days, weeks and months at a time waking only to eat or use the bathroom. This may continue for years. In between sleeping episodes, KLS sufferers may appear normal and otherwise healthy with little sign of any behavioral disorder. When awake, they may appear dazed, confused and lethargic and hypersensitive to certain noises and lights. KLS sufferers are often too tired to attend school or work and are unable to care for themselves adequately.
8. Pica Disorder
Pica is a compulsion to eat items that contain no nutritional value or may even be unsafe for consumption. People with this disorder have been known to compulsively chew on ice, flakes of dried paint, pieces of metal, soap, etc. This condition occurs most often in children and pregnant women. Although there is no single cause for Pica, it is usually temporary and often due to a nutrient deficiency such as a lack of iron or zinc. In these cases, a simple multivitamin is often sufficient as treatment. 
9. Stendhal syndrome
Stendhal syndrome (also called Florence Syndrome or hyperkulturemia) is a psychosomatic condition in which a person hyperventilates and experiences overwhelming sensations such as heart palpitations dizziness and fainting in response to viewing art or anything that is profoundly beautiful to them such as a majestic sunset. The bizarre syndrome was named after Stendhal, the pen name of 19th century French author Henri-Marie Beyle who described his hyper-emotional experience while observing Giotto’s ceiling frescoes in Florence, Italy. He wrote, “I had palpitations of the heart, what in Berlin they call ‘nerves.’ Life was drained from me. I walked with the fear of falling.” 
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image source: “Morphosis” by iNeedChemicalX