Copenhagen disease, sometimes known as Copenhagen syndrome, refers to progressive non-infectious anterior vertebral fusion, a rare childhood disease of unknown cause.

It affects the lower back, as can be seen on MRI scans. It is characterised by a lack of disc height of the vertebrae.

There are no currently known causes of this disease. There are studies currently proposing several theories of the causes which include inflammation of the adipose tissue, nervous system malfunction and endocrine malfunction. None of the theories that are currently proposed have been found viable. Since little is known about Dercum's disease, there are currently no known modes of prevention. Some hypotheses state that maintaining a healthy weight and diet can help prevent Dercum's although it has not been proven.

Dercum's disease can affect people of any gender and of any age. The majority of cases are linked to women between the ages of 45 and 60, who are overweight and postmenopausal. Due to the difficulty of diagnosis of this disease, many cases are underreported or misdiagnosed and it is difficult to understand what part of the population is affected by it the most.

Paget's disease may be caused by a slow virus infection (i.e., paramyxoviridae) present for many years before symptoms appear. Associated viral infections include respiratory syncytial virus, canine distemper virus, and the measles virus. However, recent evidence has cast some doubt upon the measles association. Laboratory contamination may have played a role in past studies linking paramyxovirus (e.g. measles) to Paget's disease.

Pagets disease of bone is the second most common metabolic bone disorder, after osteoporosis. The overall prevalence and severity of Paget's disease are decreasing; the cause for these changes is unclear. Paget's disease is rare in people less than 55 years of age, and the prevalence increases with age. Evidence from studies of autopsy results have demonstrated Paget's disease in about 3 percent of people older than 40 years of age. Paget's disease is more common in males than females. Rates of Paget's disease are about 50 percent higher in men than in women.

About 15 percent of people with Paget's disease also have a family member with the disease. In cases where the disease is familial, it is inherited in an autosomal dominant fashion, although not all people that inherit the affected version of the genes will express the disease (incomplete penetrance).

The incidence of Paget's disease varies considerably with geographic location. Paget's predominantly affects people of European descent, whereas people of African, Asian, or Indian descent are less commonly affected. Paget's disease is less common in Switzerland and Scandinavia than in the rest of Western Europe. Paget's disease is uncommon in the native populations of North and South America, Africa, Asia, and the Middle East. When an individual from these regions does develop Paget's disease, there is typically some European ancestry present.

The disease appears to be progressive in nature. The Fields twins started having problems when they were four years old. By the time they had reached the age of nine, they were having difficulty walking and needed frames to assist them with walking. Their muscles have been gradually deteriorating over time. The disease affects the twins' nerves, causing them to make involuntary muscle movements such as trembling in the hands.

The extent of the disease is still unknown as the two women are only 21. However, the disease has had no apparent effect on their brains or personalities. Doctors do not know if the disease is fatal and, if so, what the life expectancy of one with this disease is. If the cause of the disease is genetic, there is a chance that the twins could pass it on to their future children.

The twins require the use of wheelchairs for mobility and are unable to speak without the assistance of electronic speaking aids. They experience persistent and painful muscle spasms which are worsened by emotional distress. They are currently living with their parents, with the assistance of hospice workers. Doctors continue to administer tests to the twins in search of a treatment.

This disease is more common in women and an association with the gene FLT4 has been described. FLT4 codes for VEGFR-3, which is implicated in development of the lymphatic system.

Milroy's disease is also known as primary or hereditary lymphedema type 1A or early onset lymphedema.

It is a very rare disease with only about 200 cases reported in the medical literature. Milroy's disease is an autosomal dominant condition caused by a mutation in the FLT4 gene which encodes of the vascular endothelial growth factor receptor 3 (VEGFR-3) gene located on the long arm (q) on chromosome 5 (5q35.3).

In contrast to Milroy's disease (early onset lymphedema type 1A,) which typically has its onset of swelling and edema at birth or during early infancy, hereditary lymphedema type II, known as Meige disease, has its onset around the time of puberty. Meige disease is also an autosomal dominant disease. It has been linked to a mutations in the ‘forkhead’ family transcription factor (FOXC2) gene located on the long arm of chromosome 16 (16q24.3). About 2000 cases have been identified. A third type of hereditary lymphedema, that has an onset after the age of 35 is known as lymph-edema tarda.

Adiposis dolorosa, also known as Dercum's disease or Anders disease, is a rare condition characterized by generalized obesity and fatty tumors in the adipose tissue. The tumors are normally painful and found in multiples on the extremities. The cause and mechanism of Dercum's disease remains unknown. Possible causes include nervous system dysfunction, mechanical pressure on nerves, adipose tissue dysfunction, and trauma.

Dercum's disease was first described at Jefferson Medical College by neurologist Francis Xavier Dercum in 1892.

Urbach–Wiethe disease is very rare; there are fewer than 300 reported cases in medical literature. Although Urbach–Wiethe disease can be found worldwide, almost a quarter of reported diagnoses are in South Africa. Many of these are in patients of Dutch, German, and Khoisan ancestry. This high frequency is thought to be due to the founder effect. Due to its recessive genetic cause and the ability to be a carrier of the disease without symptoms, Urbach–Wiethe disease often runs in families. In some regions of South Africa, up to one in 12 individuals may be carriers of the disease. Most of the case studies involving Urbach–Wiethe disease patients involve only one to three cases and these cases are often in the same family. Due to its low incidence, it is difficult to find a large enough number of cases to adequately study the disease.

Life expectancy with Fabry disease for males was 58.2 years, compared with 74.7 years in the general population, and for females 75.4 years compared with 80.0 years in the general population, according to registry data from 2001 to 2008. The most common cause of death was cardiovascular disease, and most of those had received kidney replacements.

Milroy's disease (MD) is a familial disease characterized by lymphedema, commonly in the legs, caused by congenital abnormalities in the lymphatic system. Disruption of the normal drainage of lymph leads to fluid accumulation and hypertrophy of soft tissues. It is also known as Milroy disease, Nonne-Milroy-Meige syndrome and hereditary lymphedema.

It was named by Sir William Osler for William Milroy, a Canadian physician, who described a case in 1892, though it was first described by Rudolf Virchow in 1863.

In infantile Krabbe disease, death usually occurs in early childhood. A 2011 study found 1, 2, 3 year survival rates of 60%, 26%, and 14%, respectively. A few survived for longer and one was still alive at age 13. Patients with late-onset Krabbe disease tend to have a slower progression of the disease and live significantly longer.

With Behçet's disease as an intercurrent disease in pregnancy, the pregnancy does not have an adverse effect on the course of Behçet's disease and may possibly ameliorate its course. Still, there is a substantial variability in clinical course between patients and even for different pregnancies in the same patient. Also, the other way around, Behçet's disease confers an increased risk of pregnancy complications, miscarriage and Cesarean section.

Behçet's can cause male infertility, either as a result of the condition itself or of a side effect of concomitant medication such as Colchicine, which is known to lower sperm count.

Morbidity and mortality range from both extremes as the significance correlate with the underlying systemic disease.

Krabbe disease occurs in about one in 100,000 births. A higher incidence, about six in 1,000, has been reported in certain communities in Israel. Scandinavian countries have comparatively high rates of the disease, reported to be one in 50,000 births.

Adult-onset Still's Disease is rare and has been described all over the world. The number of new cases per year is estimated to be 1.6 per 1,000,000 population. The number of people currently affected is estimated at 1.5 cases per 100,000-1,000,000 population. Onset is most common in two age ranges, between ages 15–25 and between ages of 36–46 years.

Infants with Schindler disease tend to die within 4 years of birth, therefore, treatment for this form of the disease is mostly palliative. However, Type II Schindler disease, with its late onset of symptoms, is not characterized by neurological degeneration. There is no known cure for Schindler disease, but bone marrow transplants have been trialed, as they have been successful in curing other glycoprotein disorders.

Extramammary Paget's disease is usually seen in isolation and is associated with an underlying invasive malignancy about 12% of the time. It is associated with an underlying adnexal malignancy about 24% of the time. Paget's disease of the breast is almost always associated with an underlying invasive malignancy, i.e. breast cancer (e.g. mammary ductal carcinoma).

Urbach–Wiethe disease is typically not a life-threatening condition. The life expectancy of these patients is normal as long as the potential side effects of thickening mucosa, such as respiratory obstruction, are properly addressed. Although this may require a tracheostomy or carbon dioxide laser surgery, such steps can help ensure that individuals with Urbach–Wiethe disease are able to live a full life. Oral dimethyl sulfoxide (DMSO) has been shown to reduce skin lesions, helping to minimize discomfort for these individuals.

Fabry disease is estimated to occur in one in 40,000 to one in 120,000 live births.

The causes of Kyrle disease are unclear and can be idiopathic. The only correlation that has shown light is the frequent association with an underlying disorder, such as, diabetes mellitus, chronic renal failure, hyperlipoproteinemia, hepatic abnormalities, and congestive heart failure. However, there had been cases where Kyrle disease was seen without any conjunction with the previous mentioned disorders. Due to the causes of Kyrle disease is unknown, the best way to prevent the disease is to prevent the disorders that are usually reported in conjunction with it.

Adult-onset Still's disease (AOSD) is a form of Still's disease, a rare systemic autoinflammatory disease characterized by the classic triad of persistent high spiking fevers, joint pain, and a distinctive salmon-colored bumpy rash. The disease is considered a diagnosis of exclusion. Levels of the iron-binding protein ferritin may be elevated with this disorder. AOSD may present in a similar manner to other inflammatory diseases and to autoimmune diseases, which must be ruled out before making the diagnosis.

Prognosis is usually favorable but manifestations of the disease affecting the lungs, heart, or kidneys may occasionally cause severe life-threatening complications. It is treated first with steroids such as prednisone. Drugs that block the action of interleukin-1, such as anakinra, can be effective treatments when standard steroid treatments are insufficient.

Rosai–Dorfman disease, originally known as sinus histiocytosis with massive lymphadenopathy, is a rare disorder of unknown cause that is characterized by abundant histiocytes in the lymph nodes or other locations throughout the body.

VHL disease has an incidence of one in 36,000 births. There is over 90% penetrance by the age of 65. Age at diagnosis varies from infancy to age 60–70 years, with an average patient age at clinical diagnosis of 26 years.

It is associated with LAMP2. The status of this condition as a GSD has been disputed.