The main characteristic is that patients are extremely startled by an unexpected noise or sight. It’s not just twitching when someone sneaks up behind you. Patients with this disorder flail their arms, cry out and repeat words. First identified in some of Maine’s lumberjacks of French-Canadian origin, the odd reflex has been identified in other parts of the world, too.

Blaschko’s lines are an extremely rare and unexplained phenomenon of human anatomy first presented in 1901 by German dermatologist Alfred Blaschko. Neither a specific disease nor a predictable symptom of a disease, Blaschko’s lines are an invisible pattern built into human DNA. Many inherited and acquired diseases of the skin or mucosa manifest themselves according to these patterns, creating the visual appearance of stripes. The cause of the stripes is thought to result from mosaicism; they do not correspond to nervous, muscular, or lymphatic systems. What makes them more remarkable is that they correspond quite closely from patient to patient, usually forming a “V” shape over the spine and “S” shapes over the chest, stomach, and sides.

It is a syndrome of mental depression and suicidal tendencies, in which the patient complains of having lost everything: possessions, part of or entire body, often believing that he or she has died and is a walking corpse. This delusion is usually expanded to the degree that the patient might claim that he can smell his own rotting flesh and feel worms crawling through his skin. The latter phenomenon is a recurring experience of people chronically deprived of sleep or suffering amphetamine/cocaine psychosis. Paradoxically, being “dead” often gives the patient the nation of being immortal.

People diagnosed with Pica have an insatiable urge to eat non-food substances like dirt, paper, glue and clay. Though it is believed to be linked with mineral deficiency, health experts have found no real cause and no cure for this disorder.

A large family simply known as the “blue people” lived in the hills around Troublesome Creek in Kentucky until the 1960s. They were the blue Fugates. Most of them lived past the age of 80, with no serious illness - just blue skin. The trait was passed on from generation to generation. People with this condition have blue, plum, indigo or almost purple skin.

Frequency: About 40-50 documented cases in the world.  

Description: People who suffer from it are completely covered in lanugo hair except on the palms of their feet and hands. Lanugo is a thin, off-white hair that appears on newborns but typically disappears during the first few weeks of life. The maximum hair length that has been documented is about 25 centimeters. 

Cause: Unknown.It is probably caused by a somatic mosaicism of a lethal dominant gene. Others have suggested that it is caused by a recombination in the embryo, giving rise to three types of cells: Normal cells, cells of minimum growth and cells of excessive growth.

Frequency: Around 100 documented cases in the world. 

Description: Two twins do not separate fully, and while one twin may be healthy, the other degenerates and remains inside the other. If the host twin survives, he or she may show extra organs or swelling in the area where the parasitic fetus resides (this is typically in the abdominal region, but may also occur in the skull, *** or sacral region).  

Cause: This is an exaggerated case of Siamese twins, but the cause remains unknown. 

Frequency: Around 100 documented cases in the world. 

Description: The presence of a vestigial tail in the final zone of the sacral bone. This tail is made of connective tissue, muscles, blood vessels, nerves, skin, vertebrae and cartilage. 

Cause: Unknown, possibly caused by a genetic mutation. 

Frequency: Around 100 documented cases, although it often is not diagnosed.  

Description: People with progeria age very quickly. They appear normal at birth, but then quickly acquire wrinkles, lose their hair and suffer damage to their arteries that results in death during adolescence. 

Cause: Most of the cases of progeria are caused by mutations of autosomal dominant inheritance in the LMNA gene. This gene participates in the maintenance of nuclear stability and the organization of chromatin. It may also help to regulate genetic expression, synthesis and DNA repair. 

Frequency: About 200 documented cases in the world.  

Description: This is the syndrome that afflicted Joseph Merrick, the “Elephant Man.” It causes malformations, irregular growth of bones, hyperpigmentation and partial gigantism of your limbs.  

Cause: Unknown. It is probably caused by a somatic mosaicism of a lethal dominant gene. Others have suggested that it is caused by a recombination in the embryo, giving rise to three types of cells: Normal cells, cells of minimum growth and cells of excessive growth. 

Frequency: Between 200 and 300 cases exist around the world, but it’s estimated that one baby out of every 200,000 could have this disease. 

Description: The mechanisms of involuntary breathing do not work properly. In the most serious cases, this means a person cannot sleep without assisted ventilation. In more mild forms, patients can sleep through the night, but the lack of oxygen makes sleep difficult.   

Cause: The main cause is one or several mutations of PHOX2B gene with autosomal dominant inheritance. 

Frequency: Between 200 and 300 documented cases around the world, although it’s thought that this condition is often undiagnosed. It is estimated that one case appears in every 2 million births. 

Description: Injuries to connective tissues (muscles, ligaments, tendons) result in the formation of bone around the damaged site. Injuries may cause joints to become frozen in place, and eventually a second skeleton begins to form that restricts mobility. Surgery is not an option to remove the bone growth because the body would try to repair the area with more bone.  

Cause: Unknown. It is a disease with autosomal dominant inheritance. Several genes involved in synthesizing bone growth factors are affected.

Frequency: Around 500 documented cases in the world.  

Description: Hermaphrodites have components of both sexes, including both testicular and ovarian tissue. They may have a feminine or masculine appearance, and their external genitals may have characteristics of both sexes. 

Cause: When two zygotes that were going to be twins of different sexes become fused, it results in one individual that is, genetically, both a man and a woman. It is not know why the fusion of zygotes occurs.  

Frequency: About 80 documented cases in Spain, 200 in the U.K., and about 300 each year in Europe. 

Description: People born with Moebius Syndrome have undeveloped facial nerves that make them unable to make facial expressions. Along with being unable to smile, frown or control their blinking, they have difficulty swallowing, speaking and doing any activity that involves facial muscles. 

Cause: Unknown.Nobody knows if this disease affects nerves, brain stem or muscles. There are many and varied hypothesis, but all without proof.

Frequency: 100 documented cases in the United States.  

Description: An inability to feel pain. People with this illness may die young from injuries because they lack pain perception, and must be constantly supervised so they don’t injure themselves. 

Cause: A mutation in a gene located in the neurons associated with the reception and transmission of pain stimulus.