Riedel's thyroiditis is classified as rare. Most patients remain euthyroid, but approximately 30% of patients become hypothyroid and very few patients are hyperthyroid. It is most seen in women.

Riedel's thyroiditis is characterized by a replacement of the normal thyroid parenchyma by a dense fibrosis that invades adjacent structures of the neck and extends beyond the thyroid capsule. This makes the thyroid gland stone-hard (woody) and fixed to adjacent structures. The inflammatory process infiltrates muscles and causes symptoms of tracheal compression. Surgical treatment is required to relieve tracheal or esophageal obstruction.

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.

The increased incidence of Crohn's in the industrialized world indicates an environmental component. Crohn's is associated with an increased intake of animal protein, milk protein and an increased ratio of omega-6 to omega-3 polyunsaturated fatty acids.

Those who consume vegetable proteins appear to have a lower incidence of Crohn's disease. Consumption of fish protein has no association.

Smoking increases the risk of the return of active disease (flares). The introduction of hormonal contraception in the United States in the 1960s is associated with a dramatic increase in incidence, and one hypothesis is that these drugs work on the digestive system in ways similar to smoking. Isotretinoin is associated with Crohn's. Although stress is sometimes claimed to exacerbate Crohn's disease, there is no concrete evidence to support such claim. Dietary microparticles, such as those found in toothpaste, have been studied as they produce effects on immunity, but they were not consumed in greater amounts in patients with Crohn's.

The syndrome is rare in the United States, Africa and South America, but is common in the Middle East and Asia, suggesting a possible cause endemic to those tropical areas. A theory suggested that past exposure to lethal infectious agents might have fixed the genetic susceptibility factors to Behçet's disease in those area. It is not associated with cancer, and links with tissue-types (which are under investigation) are not certain. It also does not follow the usual pattern for autoimmune diseases. However, one study has revealed a possible connection to food allergies, particularly to dairy products. An estimated 15,000 to 20,000 Americans have been diagnosed with this disease. In the UK, it is estimated to have about 1 case for every 100,000 people. Globally, males are affected more frequently than females. In the United States, more females are affected than males.

In an epidemiologic study, 56 percent of patients with Behçet's disease developed ocular involvement at a mean age of 30. Ocular involvement was the first manifestation of Behçet's disease in 8.6 percent of patients. Ocular Behçet's disease with involvement of the optic nerve is rarely reported. Among patients with ocular Behçet's disease funduscopic findings of optic atrophy, and optic disc paleness have been identified with a frequency of 17.9 percent and 7.4 percent, respectively. Other fundoscopic findings include vascular sheathing (23.7%), retinal hemorrhage (9%), macular edema (11.3%), branch retinal vein occlusion (5.8%), and retinal edema (6.6%). However, optic atrophy was the most significant cause of visual impairment identified in 54 percent of patients with ocular Behçet's disease and permanent visual impairment.

The prevalence of this disease increases from North to South. It follows a more severe course in patients with an early age of onset particularly in patients with eye and gastrointestinal involvement.

While the exact cause is unknown, Crohn's disease seems to be due to a combination of environmental factors and genetic predisposition. Crohn's is the first genetically complex disease in which the relationship between genetic risk factors and the immune system is understood in considerable detail. Each individual risk mutation makes a small contribution to the overall risk of Crohn's (approximately 1:200). The genetic data, and direct assessment of immunity, indicates a malfunction in the innate immune system. In this view, the chronic inflammation of Crohn's is caused when the adaptive immune system tries to compensate for a deficient innate immune system.

While risk factors vary with age and gender, most of the common chronic diseases in the US are caused by dietary, lifestyle and metabolic risk factors that are also responsible for the resulting mortality. Therefore, these conditions might be prevented by behavioral changes, such as quitting smoking, adopting a healthy diet, and increasing physical activity. Social determinants are important risk factors for chronic diseases. Social factors, e.g., socioeconomic status, education level, and race/ethnicity, are a major cause for the disparities observed in the care of chronic disease. Lack of access and delay in receiving care result in worse outcomes for patients from minorities and underserved populations. Those barriers to medical care complicate patients monitoring and continuity in treatment.

In the US, Minorities and low-income populations are less likely to access and receive preventive services necessary to detect conditions at an early stage.

The majority of US health care and economic costs associated with medical conditions are for the costs of chronic diseases and conditions and associated health risk behaviors. Eighty-four percent of all health care spending in 2006 was for the 50% of the population who have one or more chronic medical conditions (CDC, 2014).

The disease is regarded as extremely rare, with an incidence (new number of cases per year) of one case per million people. The patients are predominantly male (86% in a survey of American patients), although in some countries the rate of women receiving a diagnosis of Whipple's disease has increased in recent years. It occurs predominantly in those of Caucasian ethnicity, suggesting a genetic predisposition in that population.

"T. whipplei" appears to be an environmental organism that is commonly present in the gasterointestinal tract but remains asymptomatic. Several lines of evidence suggest that some defect—inherited or acquired—in immunity is required for it to become pathogenic. The possible immunological defect may be specific for "T. whipplei", since the disease is not associated with a substantially increased risk of other infections.

The disease is usually diagnosed in middle age (median 49 years). Studies from Germany have shown that age at diagnosis has been rising since the 1960s.

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.

A growing body of evidence supports that prevention is effective in reducing the effect of chronic conditions; in particular, early detection results in less severe outcomes. Clinical preventive services include screening for the existence of the disease or predisposition to its development, counseling and immunizations against infectious agents. Despite their effectiveness, the utilization of preventive services is typically lower than for regular medical services. In contrast to their apparent cost in time and money, the benefits of preventive services are not directly perceived by patient because their effects are on the long term or might be greater for society as a whole than at the individual level.

Therefore, public health programs are important in educating the public, and promoting healthy lifestyles and awareness about chronic diseases. While those programs can benefit from funding at different levels (state, federal, private) their implementation is mostly in charge of local agencies and community-based organizations.

Studies have shown that public health programs are effective in reducing mortality rates associated to cardiovascular disease, diabetes and cancer, but the results are somewhat heterogeneous depending on the type of condition and the type of programs involved. For example, results from different approaches in cancer prevention and screening depended highly on the type of cancer.

The rising number of patient with chronic diseases has renewed the interest in prevention and its potential role in helping control costs. In 2008, the Trust for America's Health produced a report that estimated investing $10 per person annually in community-based programs of proven effectiveness and promoting healthy lifestyle (increase in physical activity, healthier diet and preventing tobacco use) could save more than $16 billion annually within a period of just five years.

Treatment is with penicillin, ampicillin, tetracycline, or co-trimoxazole for one to two years. Any treatment lasting less than a year has an approximate relapse rate of 40%. Recent expert opinion is that Whipple's disease should be treated with doxycycline with hydroxychloroquine for 12 to 18 months. Sulfonamides (sulfadiazine or sulfamethoxazole) may be added for treatment of neurological symptoms.

The cause of adult-onset Still's disease is unknown, but it presumably involves interleukin-1 (IL-1), since medications that block the action of IL-1β are effective treatments. Interleukin-18 is expressed at high levels.

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).

These differ according to the type of chronic liver disease.

- Excessive alcohol use

- Obesity

- Metabolic syndrome including raised blood lipids

- Health care professionals who are exposed to body fluids and infected blood

- Sharing infected needle and syringes

- Having unprotected sex and multiple sex partners

- Working with toxic chemicals without wearing safety clothes

- Certain prescription medications

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 of the vulva, a rare disease, may be a primary lesion or associated with adenocarcinoma originating from local organs such as the Bartholin gland, the urethra, or the rectum and thus be secondary. Patients tend to be postmenopausal.

Paget's disease of the penis may also be primary or secondary, and is even rarer than genital Paget’s disease in women. At least one case has been misdiagnosed as Bowen's disease. Isolated Paget's disease of the penis is extremely rare.

The list of conditions "associated" with chronic liver disease is extensive and can be categorised in the following way:

Viral causes

- Hepatitis B

- Hepatitis C

Cytomegalovirus (CMV), Epstein Barr virus (EBV), and yellow fever viruses cause acute hepatitis.

Toxic and drugs

- Alcoholic liver disease

- Rarely drug induced liver disease from methotrexate, amiodarone, nitrofurantoin and others

Paracetamol (acetaminophen) causes acute liver damage.

Metabolic

- Non-alcoholic fatty liver disease

- Haemochromatosis

- Wilson’s disease

Autoimmune response causes

- Primary biliary cholangitis (previously known as primary biliary cirrhosis)

- Primary sclerosing cholangitis

Other

- Right heart failure

Mortality is indirect and caused by complications. After cholangitis occurs, patients typically die within 5–10 years.

According to a recent study, the main risk factors for RA-ILD are advancing age, male sex, greater RA disease activity, rheumatoid factor (RF) positivity, and elevated titers of anticitrullinated protein antibodies such as anticyclic citrullinated peptide. Cigarette smoking also appears to increase risk of RA-ILD, especially in patients with human leukocyte antigen DRB1.

A recently published retrospective study by a team from Beijing Chao-Yang Hospital in Beijing, China, supported three of the risk factors listed for RA-ILD and identified an additional risk factor. In that study of 550 RA patients, logistic regression analysis of data collected on the 237 (43%) with ILD revealed that age, smoking, RF positivity, and elevated lactate dehydrogenase closely correlated with ILD.

Recent studies have identified risk factors for disease progression and mortality. A retrospective study of 167 patients with RA-ILD determined that the usual interstitial pneumonia (UIP) pattern on high-resolution computed tomography (HRCT) was a risk factor for progression, as were severe disease upon diagnosis and rate of change in pulmonary function test results in the first 6 months after diagnosis.

A study of 59 RA-ILD patients found no median survival difference between those with the UIP pattern and those without it. But the UIP group had more deaths, hospital admissions, need for supplemental oxygen, and decline in lung function.

The cause of Grover's disease is unknown. Suspected triggers of disease activity include heat and sweating, sunlight, and adverse reaction to medications as well as ionizing radiation, end-stage renal disease/hemodialysis, and mechanical irritation or prolonged bed rest.

Some cases of Grover's disease have been associated with medications such as sulfadoxine-pyrimethamine, ribavirin, cetuximab, and interleukin-4 [1,8-15]. One series of 300 patients with Grover's disease reported an association with other coexisting dermatoses including atopic dermatitis, contact dermatitis, and xerosis cutis. Finally, smaller series have detailed an association with pyoderma gangrenosum, bacterial and viral infections, and occasionally, malignancies.

The etiology of the condition is unknown. Possible but unproven infectious causes are "Klebsiella", polyomaviridae, Epstein–Barr virus, parvovirus B19, and human herpesvirus 6. Jilin University researchers suggested in 2017 that monocytes recruited to inflammatory lesions could produce macrophage colony-stimulating factor, which leads to a complex signal transduction, which leads to the histiocytosis characteristic of Rosai–Dorfman disease.

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.

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

Caroli disease is typically found in Asia, and diagnosed in persons under the age of 22. Cases have also been found in infants and adults. As medical imaging technology improves, diagnostic age decreases.

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.