Rare diseases are usually genetic and are therefore chronic. EURORDIS estimates that at least 80% of them have identified genetic origins. Other rare diseases are the result of infections and allergies or due to degenerative and proliferative causes.

Symptoms of some rare diseases may appear at birth or in childhood, whereas others only appear once adulthood is reached.

Research publications emphasize rare diseases that are chronic or incurable, although many short-term medical conditions are also rare diseases.

Prevalence (number of people living with a disease at a given moment), rather than incidence (number of new diagnoses in a given year), is used to describe the impact of rare diseases. The Global Genes Project estimates some 300 million people worldwide are affected by a rare disease.

The European Organization for Rare Diseases (EURORDIS) estimates that as many as 5,000 to 7,000 distinct rare diseases exist, and as much as 6% to 8% of the population of the European Union is affected by one. Only about 400 rare diseases have therapies and about 80% have a genetic component according to Rare Genomics Institute.

Rare diseases can vary in prevalence between populations, so a disease that is rare in some populations may be common in others. This is especially true of genetic diseases and infectious diseases. An example is cystic fibrosis, a genetic disease: it is rare in most parts of Asia but relatively common in Europe and in populations of European descent. In smaller communities, the founder effect can result in a disease that is very rare worldwide being prevalent within the smaller community. Many infectious diseases are prevalent in a given geographic area but rare everywhere else. Other diseases, such as many rare forms of cancer, have no apparent pattern of distribution but are simply rare. The classification of other conditions depends in part on the population being studied: All forms of cancer in children are generally considered rare, because so few children develop cancer, but the same cancer in adults may be more common.

About 40 rare diseases have a far higher prevalence in Finland; these are known collectively as Finnish heritage disease.

Hermansky–Pudlak syndrome patients, families, and caregivers are encouraged to join the NIH Rare Lung Diseases Consortium Contact Registry. This is a privacy protected site that provides up-to-date information for individuals interested in the latest scientific news, trials, and treatments related to rare lung diseases.

HPS is one of the rare lung diseases currently being studied by The Rare Lung Diseases Consortium (RLDC). The RLDC is part of the Rare Diseases Clinical Research Network (RDCRN), an initiative of the Office of Rare Diseases Research (ORDR), of the National Center for Advancing Translational Sciences (NCATS). The RLDC is dedicated to developing new diagnostics and therapeutics for patients with rare lung diseases, through collaboration between the NIH, patient organizations and clinical investigators.

Once the main cause of the disease is treated, a diet of low-fat and high-protein aliments, supplemental calcium and certain vitamins has been shown to reduce symptom effects. This diet, however, is not a cure. If the diet is stopped, the symptoms will eventually reappear.

There is no cure for GSS, nor is there any known treatment to slow the progression of the disease. However, therapies and medication are aimed at treating or slowing down the effects of the symptoms. Their goal is to try to improve the patient's quality of life as much as possible. Despite there being no cure for GSS, it is possible to undergo testing for the presence of the underlying genetic mutation. Testing for GSS involves a blood and DNA examination in order to attempt to detect the mutated gene at certain codons. If the genetic mutation is present, the patient will eventually be afflicted by GSS, and, due to the genetic nature of the disease, the offspring of the patient are predisposed to a higher risk of inheriting the mutation.

Zeichi-Ceide syndrome is a rare disease discovered in 2007. It is named after its discoverer, R.M. Zeichi-Ceide, who observed three siblings born of consanguineous parents with distinctive characteristics, including facial anomalies, large feet, mental deficiency, and occipital atretic cephalocele. The investigators suspected the symptoms were caused by autosomal recessive inheritance.

As a rare disease, Zeichi-Ceide syndrome is registered in the Online Mendelian Inheritance in Man and the U.S. National Institutes of Health's Genetic and Rare Diseases databases.

There is no cure or treatment for GSS. It can, however, be identified through genetic testing. GSS is the slowest to progress among human prion diseases. Duration of illness can range from 3 months to 13 years, with an average duration of 5 or 6 years.

The frequency is unknown, but the disease is considered to be very rare.

Fucosidosis is an extremely rare disorder first described in 1962 in two Italian siblings who showed progressive intellectual disability and neurological deterioration. The disease itself is extremely rare (less than 100 documented cases) only affecting 1:2,000,000, with most cases being occurring in Italy, Cuba, and the southwest U.S. The disease has three different types. Type 1 and 2 are considered severe, and Type 3 being a mild disease. Symptoms are highly variable with mild cases being able to live to within the third or fourth decade. Type 1 and 2 are both linked with mental retardation. Severe cases can develop life-threatening complications early in childhood.

Because the major accumulating glycoconjugate in fucosidosis patients is the blood group H-antigen, it is intriguing to speculate, but the evidence is not clear at this time, that blood type may affect the course of the disease.

Treatment: There is no treatment or way to reverse the disease. Treatment will focus on the symptoms an individual has, such as seizure medication.

- It is possible that if an individual receives a bone marrow transplant, they could receive healthy bone marrow cells which would produce normal amounts of fucosidase. But there not is enough research to prove this is an effective treatment.

Lyngstadaas Syndrome, also known as severe dental aberrations in familial steroid dehydrogenase deficiency , is a rare autosomal recessive liver disease involving an enzyme (steroid dehydrogenase) deficiency and dental anomalies. The disease is named after the Norwegian professor Ståle Petter Lyngstadaas.

Office of Rare Diseases listed Lyngstadaas syndrome as a "rare disease". This means that Lyngstadaas syndrome, or a subtype of Lyngstadaas syndrome, affects less than 200,000 people in the US population.

Orphanet, a consortium of European partners, currently defines a condition rare when if affects 1 person per 2,000. They list Lyngstadaas syndrome as a "rare disease".

Barakat syndrome, is a rare disease characterized by hypoparathyroidism, sensorineural deafness and renal disease, and hence also known as HDR syndrome. It was first described by Amin J. Barakat et al. in 1977.

Microvillus inclusion disease is thought to be extremely rare; only approximately 200 cases have been identified in children in Europe.

One patient, a teenage female living in Arizona, suddenly began to grow microvilli after thirteen years of TPN (Total Parenteral Nutrition) and Lipid dependency. She now enjoys a typical teenage diet and is seen regularly by her Gastroenterologist.

One patient from the UK was documented to achieve nutritional independence at age 3.

On 26 June 2009 a six-year-old girl diagnosed with microvillus inclusion disease became the third person in the UK to die of swine flu.

Danon disease was characterized by Moris Danon in 1981. Dr. Danon first described the disease in 2 boys with heart and skeletal muscle disease (muscle weakness), and intellectual disability.

The first case of Danon disease reported in the Middle East was a family diagnosed in the eastern region of United Arab Emirates with a new "LAMP2" mutation; discovered by the Egyptian cardiologist Dr. Mahmoud Ramadan the associate professor of Cardiology in Mansoura University (Egypt) after doing genetic analysis for all the family members in Bergamo, Italy where 6 males were diagnosed as Danon disease patients and 5 female were diagnosed as carriers; as published in "Al-Bayan" newspaper in 20 February 2016 making this family the largest one with patients and carriers of Danon disease.

Danon Disease has overlapping symptoms with another rare genetic condition called 'Pompe' disease. Microscopically, muscles from Danon Disease patients appear similar to muscles from Pompe disease patients. However, intellectual disability is rarely, if ever, a symptom of Pompe disease. Negative enzymatic or molecular genetic testing for Pompe disease can help rule out this disorder as a differential diagnosis.

Danon disease (or glycogen storage disease Type IIb) is a metabolic disorder.Danon disease is an X-linked lysosomal and glycogen storage disorder associated with hypertrophic cardiomyopathy, skeletal muscle weakness, and intellectual disability.

Treatment can involve operations to lengthen the leg bones, which involves many visits to the hospital. Other symptoms can be treated with medicine or surgery. Most female patients with the syndrome can live a long and normal life, while males have only survived in rare cases.

Waldmann disease, also known as Waldmann's disease and primary intestinal lymphangiectasia, is a rare disease characterized by enlargement of the lymph vessels supplying the lamina propria of the small intestine.

Although its prevalence is unknown, it being classified as a "rare disease" means that less than 200,000 of the population of the United States are affected by this condition and its subtypes.

Excellent for single-focus disease. With multi-focal disease 60% have a chronic course, 30% achieve remission and mortality is up to 10%.

Sandhoff disease is a rare, autosomal recessive metabolic disorder that causes progressive destruction of nerve cells in the brain and spinal cord. The disease results from mutations on chromosome 5 in the HEXB gene, critical for the lysosomal enzymes beta-N-acetylhexosaminidase A and B. Sandhoff Disease is clinically indistinguishable from Tay-Sachs Disease. The most common form, infantile Sandhoff disease, is usually fatal by early childhood.

It is nearly always fatal unless, like short bowel syndrome patients, treated with parenteral nutrition or an intestinal transplant. The patient is often classified as being in "intestinal failure" and treated with the cohort of patients known as "short bowel syndrome" patients.

Guidelines for management of patients up to 18 years with Langerhans cell histiocytosis has been suggested. Treatment is guided by extent of disease. Solitary bone lesion may be amenable through excision or limited radiation, dosage of 5-10 Gys for children, 24-30 Gys for adults. However systemic diseases often require chemotherapy. Use of systemic steroid is common, singly or adjunct to chemotherapy. Local steroid cream is applied to skin lesions. Endocrine deficiency often require lifelong supplement e.g. desmopressin for diabetes insipidus which can be applied as nasal drop. Chemotherapeutic agents such as alkylating agents, antimetabolites, vinca alkaloids either singly or in combination can lead to complete remission in diffuse disease.

Microcephalic osteodysplastic primordial dwarfism type II (MOPD II) is listed as a "rare disease" by the Office of Rare Diseases (ORD) of the National Institutes of Health (NIH). This means that MOPD, or a subtype of MOPD, affects less than 200,000 people in the US population and a form of dwarfism associated with brain and skeletal abnormalities.

It was characterized in 1982.

It is associated with "PCNT".

Current treatment options include:

- Surgical debulking

- High-dose Corticosteroid therapy

- Cyclosporin

- Interferon-α

- Chemotherapy

- Vemurafenib. It would appear that approximately half these patients harbor point mutations of the BRAF gene at codon 600 substituting the amino acid glutamine for valine. Vemurafenib, an oral FDA approved targeted agent to the BRAF protein for melanoma, shows dramatic activity in patients Erdheim–Chester disease whose tumor contains the same mutation. In 2017 the US FDA approved vemurafenib for this indication.

- Radiation therapy

All current treatments have had varying degrees of success.

The vinca alkaloids and anthracyclines have been used most commonly in ECD treatment.