A 2009 study which followed 189 patients found no excess mortality despite the increased risk of pancreatic cancer.

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.

treatment of HP resemble that of chronic pancreatitis of other causes. Treatment focuses on enzyme and nutritional supplementation, pain management, pancreatic diabetes, and local organ complications, such as pseudocysts, bile duct or duodenal obstruction.(PMC1774562)

The course of HPS has been mild in rare instances of the disorder, however, the general prognosis is still considered to be poor.

The disease can cause dysfunctions of the lungs, intestine, kidneys, and heart. The major complication of most forms of the disorder is pulmonary fibrosis, which typically exhibits in patients ages 40–50 years. This is a fatal complication seen in many forms of HPS, and is the usual cause of death from the disorder. HPS patients who develop pulmonary fibrosis typically have type 1 or type 4.

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.

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.

Genetic disorders may also be complex, multifactorial, or polygenic, meaning they are likely associated with the effects of multiple genes in combination with lifestyles and environmental factors. Multifactorial disorders include heart disease and diabetes. Although complex disorders often cluster in families, they do not have a clear-cut pattern of inheritance. This makes it difficult to determine a person’s risk of inheriting or passing on these disorders. Complex disorders are also difficult to study and treat, because the specific factors that cause most of these disorders have not yet been identified. Studies which aim to identify the cause of complex disorders can use several methodological approaches to determine genotype-phenotype associations. One method, the genotype-first approach, starts by identifying genetic variants within patients and then determining the associated clinical manifestations. This is opposed to the more traditional phenotype-first approach, and may identify causal factors that have previously been obscured by clinical heterogeneity, penetrance, and expressivity.

On a pedigree, polygenic diseases do tend to "run in families", but the inheritance does not fit simple patterns as with Mendelian diseases. But this does not mean that the genes cannot eventually be located and studied. There is also a strong environmental component to many of them (e.g., blood pressure).

- asthma

- autoimmune diseases such as multiple sclerosis

- cancers

- ciliopathies

- cleft palate

- diabetes

- heart disease

- hypertension

- inflammatory bowel disease

- intellectual disability

- mood disorder

- obesity

- refractive error

- infertility

Melkersson–Rosenthal syndrome may recur intermittently after its first appearance. It can become a chronic disorder. Follow-up care should exclude the development of Crohn's disease or sarcoidosis.

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

Not to be confused with Rosenthal syndrome a.k.a hemophilia C which is caused by clotting factor XI deficiency. Only genetic causation is established as it is associated with twins and family members.

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.

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.

Due to the wide range of genetic disorders that are presently known, diagnosis of a genetic disorder is widely varied and dependent of the disorder. Most genetic disorders are diagnosed at birth or during early childhood, however some, such as Huntington's disease, can escape detection until the patient is well into adulthood.

The basic aspects of a genetic disorder rests on the inheritance of genetic material. With an in depth family history, it is possible to anticipate possible disorders in children which direct medical professionals to specific tests depending on the disorder and allow parents the chance to prepare for potential lifestyle changes, anticipate the possibility of stillbirth, or contemplate termination. Prenatal diagnosis can detect the presence of characteristic abnormalities in fetal development through ultrasound, or detect the presence of characteristic substances via invasive procedures which involve inserting probes or needles into the uterus such as in amniocentesis.

CVID has an estimated prevalence of about 1:50,000 in caucasians. The disease seems to be less prevalent amongst Asians and African-Americans. Males and females are equally affected; however, among children, boys predominate. A recent study of people in European with primary immunodeficiencies found that 30% had CVID, as opposed to a different immunodeficiency. 10-25% of people inherited the disease, typically through autosomal-dominant inheritance. Given the rarity of the disease, it is not yet possible to generalize on disease prevalence among ethnic and racial groups. CVID shortens the life-span; the median age of death for men and women is 42 and 44 years old, respectively. Those people with accompanying disorders had the worst prognosis and those people with CVID only had frequent infections had the longest survival rates, with life expectancy almost equalling that of the general UK population. Additionally, people with CVID with one or more noninfectious complications have an 11 times higher risk of death as compared to people with only infections.

Opitz G/BBB Syndrome is a rare genetic condition caused by one of two major types of mutations: MID1 mutation on the short (p) arm of the X chromosome or a mutation of the 22q11.2 gene on the 22nd chromosome. Since it is a genetic disease, it is an inherited condition. However, there is an extremely wide variability in how the disease presents itself.

In terms of prevention, several researchers strongly suggest prenatal testing for at-risk pregnancies if a MID1 mutation has been identified in a family member. Doctors can perform a fetal sex test through chromosome analysis and then screen the DNA for any mutations causing the disease. Knowing that a child may be born with Opitz G/BBB syndrome could help physicians prepare for the child’s needs and the family prepare emotionally. Furthermore, genetic counseling for young adults that are affected, are carriers or are at risk of carrying is strongly suggested, as well (Meroni, Opitz G/BBB syndrome, 2012). Current research suggests that the cause is genetic and no known environmental risk factors have been documented. The only education for prevention suggested is genetic testing for at-risk young adults when a mutation is found or suspected in a family member.

Current research is aimed at studying large cohorts of people with CVID in an attempt to better understand age of onset, as well as mechanism, genetic factors, and progression of the disease.

Funding for research in the US is provided by the National Institutes of Health. Key research in the UK was previously funded by the Primary Immunodeficiency Association (PiA) until its closure in January 2012, and funding is raised through the annual Jeans for Genes campaign. Current efforts are aimed at studying the following:

- Causes of complications. Little is known about why such diverse complications arise during treatment

- Underlying genetic factors. Though many polymorphisms and mutations have been identified, their respective roles in CVID development are poorly understood, and not represented in all people with CVID.

- Finding new ways to study CVID. Given that CVID arises from more than one gene, gene knock-out methods are unlikely to be helpful. It is necessary to seek out disease related polymorphisms by screening large populations of people with CVID, but this is challenging given the rarity of the disease.

Most cases are X-linked recessive but there may be as many as three types. As well as a classical X-linked form, there is another type where females are partially affected and another where females have full IFAP symptoms. The gene or genes causing this disease are not known.

IFAP syndrome is an extremely rare genetic syndrome. It is also known as Ichthyosis follicularis, alopecia, and photophobia syndrome or simply ichthyosis follicularis. It is extremely rare: there were only 10 known cases (all male) in 1998.

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

Congenital chloride diarrhea (CCD, also congenital chloridorrhea or Darrow Gamble syndrome) is a genetic disorder due to an autosomal recessive mutation on chromosome 7. The mutation is in downregulated-in-adenoma (DRA), a gene that encodes a membrane protein of intestinal cells. The protein belongs to the solute carrier 26 family of membrane transport proteins. More than 20 mutations in the gene are known to date. A rare disease, CCD occurs in all parts of the world but is more common in some populations with genetic founder effects, most notably in Finland.

Cryopyrin-associated periodic syndrome (CAPS) is a group of rare, heterogeneous autoinflammatory disease characterized by interleukin 1β-mediated systemic inflammation and clinical symptoms involving skin, joints, central nervous system, and eyes. It encompasses a spectrum of three clinically overlapping autoinflammatory syndromes including familial cold autoinflammatory syndrome (FCAS, formerly termed familial cold-induced urticaria), the Muckle–Wells syndrome (MWS), and neonatal-onset multisystem inflammatory disease (NOMID, also called chronic infantile neurologic cutaneous and articular syndrome or CINCA) that were originally thought to be distinct entities, but in fact share a single genetic mutation and pathogenic pathway.

Although the genetic cause of Danon Disease is known, the mechanism of disease is not well understood. Danon disease involves a genetic defect (mutation) in a gene called LAMP2, which results in a change to the normal protein structure. While the function of the "LAMP2" gene is not well understood, it is known that LAMP2 protein is primarily located in small structures within cells called lysosomes.

Kikuchi-Fujimoto disease (KFD) is a rare, self-limiting disorder that typically affects the cervical lymph nodes. Recognition of this condition is crucial, especially because it can easily be mistaken for tuberculosis, lymphoma, or even adenocarcinoma. Awareness of this disorder helps prevent misdiagnosis and inappropriate treatment.

Kikuchi's disease is a very rare disease mainly seen in Japan. Isolated cases are reported in North America, Europe, and Asia. It is mainly a disease of young adults (20–30 years), with a slight bias towards females. The cause of this disease is not known, although infectious and autoimmune causes have been proposed. The course of the disease is generally benign and self-limiting. Lymph node enlargmeent usually resolves over several weeks to six months. Recurrence rate is about 3%. Death from Kikuchi disease is extremely rare and usually occurs due to liver, respiratory, or heart failure.

Although no cause has been officially confirmed, researchers speculate the disease might result from a genetic mutation that sporadically occurs for unknown reasons.

Observations leading to the characterization of the SLC26 family were based on research on rare human diseases. Three rare recessive diseases in humans have been shown to be caused by genes of this family. Diastrophic dysplasia, congenital chloride diarrhea, and Pendred syndrome are caused by the highly related genes SLC26A2 (first called DTDST), SLC26A3 (first called CLD or DRA), and SLC26A4 (first called PDS), respectively. Two of these diseases, diastrophic dysplasia and congenital chloride diarrhea, are Finnish heritage diseases.