A rare disease is generally defined as one that affects less than five in 10,000 of the general population. There are currently an estimated 6,000 to 8,000 known rare diseases and the Rare Disease UK site states that around five new rare diseases are described in medical literature each week.

Rare diseases are often chronic and life-threatening, and 80% of them have a genetic component. These diseases can be single gene, multifactorial, chromosomal or non-genetic. Additionally, 75% of rare diseases affect children.

In the UK, a single rare disease may affect up to about 30,000 people. The vast majority of rare diseases will affect far fewer than this – some will affect only a handful, or even a single person in a whole country. Here we take a look at five rare diseases that are relatively unknown and quite bizarre.

1. Fibrous dysplasia

This disorder results in scar-like tissues, which are fibrous, developing in place of normal bone tissue. The result is the sufferer would suffer from a weakened bone, and makes it more susceptible to deformities and fractures. Most cases of fibrous dysplasia occur in a single bone — most commonly the skull or the arms or legs. Occurring most often in adolescents and young adults, those with a more severe form of this illness would develop symptoms before the age of 10. It is caused by genetic factors, and there is no current cure; treatment focuses on relieving signs and symptoms.

2. Epidermodysplasia verruciformis

Epidermodysplasia verruciformis is a rare autosomal recessive inherited skin disorder characterised by the appearance of flat wart-like lesions, occurring anywhere on the body. It is caused by the same virus that causes cervical cancer – the human papillomavirus (HPV). With up to 60 differing genotypes of HPV, some types cause skin cancer while other types cause lesions. Patients with epidermodysplasia verruciformis have an abnormal or impaired immune response to HPV or wart viruses. Why this disorder would turn cancerous is yet unknown, but it is suspected to be caused by UV rays, and the type of HPV infection.

3. Macrodactyly

Macrodactyly is an uncommon condition where a baby’s toes or fingers are abnormally large due to the overgrowth of the underlying bone and soft tissue. Sufferers are born with it. The condition affects hands more often than feet, and most of the time, only one hand or foot is affected, but multiple digits are affected. This condition may often be diagnosed together with syndactyly, where two fingers or toes are fused together. Generally, it is a benign condition, but is deforming and is cosmetically displeasing. However, sufferers can draw inspiration from success stories such as that of US women’s hockey star Amber Galles who has successfully lived with the condition her whole life.

4. Diprosopus

This “double-faced syndrome” is a condition so rare, there have only been 35 such cases in human history – with none surviving, and many more are stillborn. The affected baby would be born with two faced, medically known as ‘craniofacial duplication’. The most severe cases reported total duplication of facial features; a less serious condition exists whereby the nose is duplicated, and the eyes are spaced far apart.

An Australian woman gave birth to conjoined twins with a rare form of diprosopus in 2014. They share a body and organs, with only separate heads as they are connected at the brain stem. Thus, this case is classified as a rare form of diprosopus – conjoined twinning.

5. Noma

Noma is specific type of gangrene that destroys the mucous membranes of the mouth and genitals, amongst other tissues. Researchers have not been able to identify what exactly causes this rare disorder, although it is suspected to be attributable towards a certain type of bacteria. Noma primarily affects malnourished children, aged two to five, in areas where sanitation and cleanliness are lacking. Other risk factors include prior infection of measles, scarlet fever, tuberculosis, or cancer, or those with a compromised immune system. MIMS

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http://www.mayoclinic.org/diseases-conditions/fibrous-dysplasia/basics/definition/con-20032196 https://rarediseases.org/rare-diseases/fibrous-dysplasia/