This condition has been linked to mutations in the ribosomal GTPase BMS1 gene.

Aplasia cutis congenita (ACC) is a rare disorder characterized by congenital absence of skin. Frieden classified ACC in 1986 into 9 groups on the basis of location of the lesions and associated congenital anomalies. The scalp is the most commonly involved area with lesser involvement of trunk and extremities. Frieden classified ACC with fetus papyraceus as type 5. This type presents as truncal ACC with symmetrical absence of skin in stellate or butterfly pattern with or without involvement of proximal limbs.]It is the most common congenital cicatricial alopecia, and is a congenital focal absence of epidermis with or without evidence of other layers of the skin.

The exact etiology of ACC is still unclear but intrauterine infection by varicella or herpes virus, drugs such as methimazole, misoprostol, valproate, cocaine, marijuana etc., fetus papyraceus, feto-fetal transfusion, vascular coagulation defects, amniotic membrane adherence, abnormal elastic fiber biomechanical forces and trauma are implicated. It can be associated with Johanson-Blizzard syndrome, Adams-Oliver syndrome, trisomy 13, and Wolf-Hirschhorn syndrome.

It can also seen with exposure to methimazole and carbimazole in utero. This dermatological manifestation has been linked to Peptidase D haploinsufficiency and a deletion in Chromosome 19.

In utero exposure to cocaine and other street drugs can lead to agenesis of corpus callosum.

Until recently, the medical literature did not indicate a connection among many genetic disorders, both genetic syndromes and genetic diseases, that are now being found to be related. As a result of new genetic research, some of these are, in fact, highly related in their root cause despite the widely varying symptoms apparent on clinical examination. Agenesis of the corpus callosum is one such disease, part of an emerging class of diseases called ciliopathies. The underlying cause may be a dysfunctional molecular mechanism in the primary cilia structures of the cell organelles that are present in many cellular types throughout the human body. The cilia defects adversely affect "numerous critical developmental signaling pathways" essential to cellular development and thus offer a plausible hypothesis for the often multi-symptom nature of a large set of syndromes and diseases. Known ciliopathies include primary ciliary dyskinesia, Bardet–Biedl syndrome, polycystic kidney and liver disease, nephronophthisis, Alström syndrome, Meckel–Gruber syndrome, and some forms of retinal degeneration.

This is a very rare neoplasm accounting for approximately 0.0003% of all tumors and about 2.5% of all external ear neoplasms. There is a wide age range at initial presentation, although the mean age is about 50 years of age. Females are affected slightly more often (1.5:1).

Wide, radical, complete surgical excision is the treatment of choice, with free surgical margins to achieve the best outcome and lowest chance of recurrence. Radiation is only used for palliation. In general, there is a good prognosis, although approximately 50% of patients die from disease within 3–10 years of presentation.

Adenoid cystic carcinoma (sometimes referred to as adenocyst, malignant cylindroma, adenocystic, adenoidcystic, ACC or AdCC.) is a rare type of cancer that can exist in many different body sites. This tumor most often occurs in the salivary glands, but it can also be found in many anatomic sites, including the breast, lacrimal gland, lung, brain, bartholin gland, trachea, and the paranasal sinuses.

It is the third most common malignant salivary gland tumor overall (after mucoepidermoid carcinoma and polymorphous low grade adenocarcinoma). It represents 28% of malignant submandibular gland tumors, making it the single most common malignant salivary gland tumor in this region. Patients may survive for years with metastases because this tumor is generally well-differentiated and slow growing. In a 1999 study of a cohort of 160 ACC patients, disease specific survival was 89% at 5 years but only 40% at 15 years, reflecting deaths from late-occurring metastatic disease.

Primary treatment for this cancer, regardless of body site, is surgical removal with clean margins. This surgery can prove challenging in the head and neck region due to this tumour's tendency to spread along nerve tracts. Adjuvant or palliative radiotherapy is commonly given following surgery. For advanced major and minor salivary gland tumors that are inoperable, recurrent, or exhibit gross residual disease after surgery, fast neutron therapy is widely regarded as the most effective form of treatment.

Chemotherapy is used for metastatic disease. Chemotherapy is considered on a case by case basis, as there is limited trial data on the positive effects of chemotherapy. Clinical studies are ongoing, however.

Acinar cell carcinoma of the pancreas, also acinar cell carcinoma, is a rare malignant exocrine tumour of the pancreas. It represents 5% of all exocrine tumours of the pancreas, making it the second most common type of pancreatic cancer. It is abbreviated ACC. It typically has a guarded prognosis.

The main etiologic factor of adrenocortical cancer is unknown. Families with Li–Fraumeni syndrome have increased risk. The p53, retinoblastoma protein (RB) tumor suppressor genes located on chromosomes 17p, 13q respectively, may be changed. The genes h19, insulin-like growth factor II (IGF-II), p57 are important for fetal growth and development. They are located on chromosome 11p. Expression of the h19 gene is markedly reduced in both nonfunctioning and functioning adrenal cortical carcinomas, especially in tumors producing cortisol and aldosterone. There is also a loss of activity of the p57 gene product in virilizing adenomas and adrenal cortical carcinomas. In contrast, IGF-II gene expression has been shown to be high in adrenal cortical carcinomas. Finally, c-myc gene expression is relatively high in neoplasms, and it is often linked to poor prognosis.

Bilateral adrenocortical tumors are less common than unilateral. The majority of bilateral tumours can be distinguished according to size and aspect of the nodules: primary pigmented nodular adrenocortical disease which can be sporadic or part of Carney complex and primary bilateral macro nodular adrenal hyperplasia.

ACC, generally, carries a poor prognosis and is unlike most tumours of the adrenal cortex, which are benign (adenomas) and only occasionally cause Cushing's syndrome. Five-year disease-free survival for a complete resection of a stage I–III ACC is approximately 30%.

The most important prognostic factors are age of the patient and stage of the tumor.

Poor prognostic factors: mitotic activity, venous invasion, weight of 50g+; diameter of 6.5 cm+, Ki-67/MIB1 labeling index of 4%+, p53+.

The disease is more common in men than women and the average age at diagnosis is about 60.

Symptoms are often non-specific and include weight loss. A classic presentation, found in around 15% of cases includes subcutaneous nodules (due to fat necrosis) and arthralgias, caused by release of lipase.

An adrenal tumor or adrenal mass is any benign or malignant neoplasms of the adrenal gland, several of which are notable for their tendency to overproduce endocrine hormones. Adrenal cancer is the presence of malignant adrenal tumors, and includes neuroblastoma, adrenocortical carcinoma and some adrenal pheochromocytomas. Most adrenal pheochromocytomas and all adrenocortical adenomas are benign tumors, which do not metastasize or invade nearby tissues, but may cause significant health problems by unbalancing hormones.

The adrenal cortex is composed of three distinct layers of endocrine cells which produce critical steroid hormones. These include the glucocorticoids which are critical for regulation of blood sugar and the immune system, as well as response to physiological stress, the mineralcorticoid aldosterone, which regulates blood pressure and kidney function, and certain sex hormones. Both benign and malignant tumors of the adrenal cortex may produce steroid hormones, with important clinical consequences.

Pellagra can develop according to several mechanisms, classically as a result of niacin (vitamin B3) deficiency, which results in decreased NAD production leading to most of the pathology (since NAD and its phosphorylated NADP form are cofactors required in many body processes, the pathological impact of pellagra is broad and results in death if not treated).

The first mechanism is simple dietary lack of niacin. Second, it may result from deficiency of tryptophan, an essential amino acid found in meat, poultry, fish, eggs, and peanuts that the body converts into niacin. Third, it may be caused by excess leucine, as it inhibits quinolinate phosphoribosyl transferase (QPRT) and inhibits the formation of Niacin or Nicotinic acid to Nicotinamide mononucleotide (NMN) causing pellegra like symptoms to occur.

Some conditions can prevent the absorption of dietary niacin or tryptophan and lead to pellagra. Inflammation of the jejunum or ileum can prevent nutrient absorption, leading to pellagra, and this can in turn be caused by Crohn's disease. Gastroenterostomy can also cause pellagra. Chronic alcoholism can also cause poor absorption which combines with a diet already low in niacin and tryptophan to produce pellagra. Hartnup disease is a genetic disorder that reduces tryptophan absorption, leading to pellagra.

Alterations in protein metabolism may also produce pellagra-like symptoms. An example is carcinoid syndrome, a disease in which neuroendocrine tumors along the GI tract use tryptophan as the source for serotonin production, which limits the available tryptophan for niacin synthesis. In normal patients, only one percent of dietary tryptophan is converted to serotonin; however, in patients with carcinoid syndrome, this value may increase to 70%. Carcinoid syndrome thus may produce niacin deficiency and clinical manifestations of pellagra. Anti-tuberculosis medication tends to bind to vitamin B and reduce niacin synthesis, since B (aka pyridoxine) is a required cofactor in the tryptophan-to-niacin reaction.

Several therapeutic drugs can provoke pellagra. These include the antibiotics isoniazid, which decreases available B by binding to it and making it inactive, so it cannot be used in niacin synthesis, and chloramphenicol; the anti-cancer agent fluorouracil; and the immunosuppressant mercaptopurine.

Pellagra is a disease caused by a lack of the vitamin niacin (vitamin B). Symptoms include inflamed skin, diarrhea, dementia, and sores in the mouth. Areas of the skin exposed to either sunlight or friction are typically affected first. Over time affected skin may become darker, stiff, begin to peel, or bleed.

There are two main types of pellagra, primary and secondary. Primary pellagra is due to a diet that does not contain enough niacin and tryptophan. Secondary pellagra is due to a poor ability to use the niacin within the diet. This can occur as a result of alcoholism, long term diarrhea, carcinoid syndrome, Hartnup disease, and a number of medications such as isoniazid. Diagnosis is typically based on symptoms and may be assisted by urine testing.

Treatment is with either niacin or nicotinamide supplementation. Improvements typically begin within a couple of days. General improvements in diet are also frequently recommended. Decreasing sun exposure via sunscreen and proper clothing is important while the skin heals. Without treatment death may occur. It occurs most commonly in the developing world, specifically sub-Saharan Africa.

Mind-blindness is a cognitive disorder where an individual is unable to attribute mental states to others. As a result of this kind of social and empathetic cognitive deficit, the individual is incapable in putting himself "into someone else's shoes" and cannot conceptualize, understand or predict knowledge, thoughts and beliefs, emotions, feelings and desires, behaviour, actions and intentions of another person. Such an ability to develop a mental awareness of what is in the other minds is known as the theory of mind (ToM), and the "Mind-blindness" Theory asserts that children who delay in this development often are or will be autistic and Asperger's syndrome (AS) patients. In addition to autism and AS, ToM and mind-blindness research has recently been extended to other disorders such as schizophrenia, dementia, bipolar disorders, antisocial personality disorders as well as normal aging.

Anorexia nervosa has been increasingly diagnosed since 1950; the increase has been linked to vulnerability and internalization of body ideals. People in professions where there is a particular social pressure to be thin (such as models and dancers) were more likely to develop anorexia, and those with anorexia have much higher contact with cultural sources that promote weight loss. This trend can also be observed for people who partake in certain sports, such as jockeys and wrestlers. There is a higher incidence and prevalence of anorexia nervosa in sports with an emphasis on aesthetics, where low body fat is advantageous, and sports in which one has to make weight for competition. Family dynamics can play big part in the cause of anorexia. When there is a constant pressure from people to be thin, teasing, bullying can cause low self-esteem and other psychological symptoms.

Bipolar disorder can cause suicidal ideation that leads to suicidal attempts. Individuals whose bipolar disorder begins with a depressive or mixed affective episode seem to have a poorer prognosis and an increased risk of suicide. One out of two people with bipolar disorder attempt suicide at least once during their lifetime and many attempts are successfully completed. The annual average suicide rate is 0.4 percent, which is 10–20 times that of the general population. The standardized mortality ratio from suicide in bipolar disorder is between 18 and 25. The lifetime risk of suicide has been estimated to be as high as 20 percent in those with bipolar disorder.

Anorexia nervosa is highly heritable. Twin studies have shown a heritability rate of between 28 and 58%. First degree relative of those with anorexia have roughly 12 times the risk of developing anorexia. Association studies have been performed, studying 128 different polymorphisms related to 43 genes including genes involved in regulation of eating behavior, motivation and reward mechanics, personality traits and emotion. Consistent associations have been identified for polymorphisms associated with agouti-related peptide, brain derived neurotrophic factor, catechol-o-methyl transferase, SK3 and opioid receptor delta-1. Epigenetic modifications, such as DNA methylation, may contribute to the development or maintenance of anorexia nervosa, though clinical research in this area is in its infancy.

The causes of bipolar disorder likely vary between individuals and the exact mechanism underlying the disorder remains unclear. Genetic influences are believed to account for 60–80 percent of the risk of developing the disorder indicating a strong hereditary component. The overall heritability of the bipolar spectrum has been estimated at 0.71. Twin studies have been limited by relatively small sample sizes but have indicated a substantial genetic contribution, as well as environmental influence. For bipolar disorder type I, the rate at which identical twins (same genes) will both have bipolar disorder type I (concordance) is estimated at around 40 percent, compared to about 5 percent in fraternal twins. A combination of bipolar I, II, and cyclothymia similarly produced rates of 42 percent and 11 percent (identical and fraternal twins, respectively), with a relatively lower ratio for bipolar II that likely reflects heterogeneity. There is overlap with major (unipolar) depression and if this is also counted in the co-twin the concordance with bipolar disorder rises to 67 percent in identical twins and 19 percent in fraternal twins. The relatively low concordance between fraternal twins brought up together suggests that shared family environmental effects are limited, although the ability to detect them has been limited by small sample sizes.

The early stages of Alzheimer's disease are difficult to diagnose. A definitive diagnosis is usually made once cognitive impairment compromises daily living activities, although the person may still be living independently. The symptoms will progress from mild cognitive problems, such as memory loss through increasing stages of cognitive and non-cognitive disturbances, eliminating any possibility of independent living, especially in the late stages of the disease.

Life expectancy of people with AD is less. Following diagnosis it typically ranges from three to ten years.

Fewer than 3% of people live more than fourteen years. Disease features significantly associated with reduced survival are an increased severity of cognitive impairment, decreased functional level, history of falls, and disturbances in the neurological examination. Other coincident diseases such as heart problems, diabetes or history of alcohol abuse are also related with shortened survival. While the earlier the age at onset the higher the total survival years, life expectancy is particularly reduced when compared to the healthy population among those who are younger. Men have a less favourable survival prognosis than women.

Pneumonia and dehydration are the most frequent immediate causes of death brought by AD, while cancer is a less frequent cause of death than in the general population.

Two main measures are used in epidemiological studies: incidence and prevalence. Incidence is the number of new cases per unit of person–time at risk (usually number of new cases per thousand person–years); while prevalence is the total number of cases of the disease in the population at any given time.

Regarding incidence, cohort longitudinal studies (studies where a disease-free population is followed over the years) provide rates between 10 and 15 per thousand person–years for all dementias and 5–8 for AD, which means that half of new dementia cases each year are AD. Advancing age is a primary risk factor for the disease and incidence rates are not equal for all ages: every five years after the age of 65, the risk of acquiring the disease approximately doubles, increasing from 3 to as much as 69 per thousand person years. There are also sex differences in the incidence rates, women having a higher risk of developing AD particularly in the population older than 85. The risk of dying from Alzheimer's disease is 26% higher among the non-Hispanic white population than among the non-Hispanic black population, whereas the Hispanic population has a 30% lower risk than the non-Hispanic white population.

Prevalence of AD in populations is dependent upon different factors including incidence and survival. Since the incidence of AD increases with age, it is particularly important to include the mean age of the population of interest. In the United States, Alzheimer prevalence was estimated to be 1.6% in 2000 both overall and in the 65–74 age group, with the rate increasing to 19% in the 75–84 group and to 42% in the greater than 84 group. Prevalence rates in less developed regions are lower. The World Health Organization estimated that in 2005, 0.379% of people worldwide had dementia, and that the prevalence would increase to 0.441% in 2015 and to 0.556% in 2030. Other studies have reached similar conclusions. Another study estimated that in 2006, 0.40% of the world population (range 0.17–0.89%; absolute number , range ) were afflicted by AD, and that the prevalence rate would triple and the absolute number would quadruple by 2050.

In chiropractic, vertebral subluxation is a purported misalignment of the spinal column, not necessarily visible on X-rays, leading to a set of signs and symptoms sometimes termed vertebral subluxation complex. It has no biomedical basis, lacks clinical meaningfulness, and is categorized as pseudoscientific by leading chiropractic authorities. Traditionally, the "specific focus of chiropractic practice" is the chiropractic subluxation and historical chiropractic practice assumes that a vertebral subluxation or spinal joint dysfunction interferes with the body's function and its innate intelligence, as promulgated by D. D. Palmer, the inventor of chiropractic.

The chiropractic subluxation is the heart of the split between "straight" and "mixer" chiropractors. Straight chiropractors continue to follow Palmer's vitalistic tradition, claiming that subluxation has considerable health effects and also adding a visceral component to the definition, while mixers, as exemplified by the United Kingdom's General Chiropractic Council, consider it a historical concept with no evidence identifying it as the cause of disease.

Within the chiropractic tradition, a vertebral subluxation complex is believed to be a dysfunctional biomechanical spinal segment which actively alters neurological function, which in turn, is believed to lead to neuromusculoskeletal and visceral disorders. The WHO acknowledges this difference between the medical and chiropractic definitions of a subluxation: medical doctors only refer to "significant structural displacements" as subluxations, whereas chiropractors suggest that a dysfunctional segment, whether displaced significantly or not, should be referred to as a subluxation. This difference has been noted in the proceedings of the chiropractic profession's "Mercy Center Consensus Conference": "The chiropractic profession refers to this concept as a 'subluxation'. This use of the word "subluxation" should not be confused with the term's precise anatomic usage, which considers only the anatomical relationships."

The chiropractic vertebral subluxation complex has been a source of controversy since its inception in 1895 due to the lack of empirical evidence for its existence, its metaphysical origins, and claims of its far reaching effects on health and disease. Although some chiropractic associations and colleges support the concept of subluxation, many in the chiropractic profession reject it and shun the use of this term as a diagnosis. In the United States and in Canada the term "nonallopathic lesion" may be used in place of "subluxation".

A 2009 review concluded that epidemiologic evidence does not support the chiropractic subluxation, concluding:

In 2015, internationally accredited chiropractic colleges from Bournemouth University, University of South Wales, University of Southern Denmark, University of Zürich, Institut Franco-Européen de Chiropraxie, and University of Johannesburg made an open statement which included: "The teaching of the vertebral subluxation complex as a vitalistic construct that claims that it is the cause of disease is unsupported by evidence. Its inclusion in a modern chiropractic curriculum in anything other than an historic context is therefore inappropriate and unnecessary".

It has been proposed that a vertebral subluxation can negatively affect general health by altering the neurological communication between the brain, spinal cord and peripheral nervous system. Although individuals may not always be symptomatic, straight chiropractors believe that the presence of vertebral subluxation is in itself justification for correction via spinal adjustment.

Chiropractic treatment of vertebral subluxation focuses on delivering a chiropractic adjustment which is a high velocity low amplitude (HVLA) thrust to the dysfunctional spinal segments to help correct the chiropractic subluxation complex. Spinal adjustment is the primary procedure used by chiropractors in the adjustment. Adjustment/manipulation has been shown to help with low back pain, neck pain and tension type headaches, but further studies are inconclusive on the use of spinal manipulation outside the treatment of neuromusculoskeletal disorders.