Heterogeneous medical condition in medicine are those medical conditions which have several etiologies, like hepatitis or diabetes. Medical conditions are normally defined pathologically (liver inflammation) or clinically (excessive urination) and not etiologically, and therefore it is normal to have more than one cause for them. The word is used as an opposition to homogeneous, meaning that given a group of patients, the disease is the same for all of them.

When a condition is heterogeneous, it is normally divided in endotypes.

An aging-associated disease is a disease that is most often seen with increasing frequency with increasing senescence. Essentially, aging-associated diseases are complications arising from senescence. Age-associated diseases are to be distinguished from the aging process itself because all adult animals age, save for a few rare exceptions, but not all adult animals experience all age-associated diseases. Aging-associated diseases do not refer to age-specific diseases, such as the childhood diseases chicken pox and measles. "Aging-associated disease" is used here to mean "diseases of the elderly". Nor should aging-associated diseases be confused with accelerated aging diseases, all of which are genetic disorders.

Examples of aging-associated diseases are atherosclerosis and cardiovascular disease, cancer, arthritis, cataracts, osteoporosis, type 2 diabetes, hypertension and Alzheimer's disease. The incidence of all of these diseases increases rapidly with aging (increases exponentially with age, in the case of cancer).

Of the roughly 150,000 people who die each day across the globe, about two thirds—100,000 per day—die of age-related causes. In industrialized nations, the proportion is higher, reaching 90%.

Disease presentation varies widely from patient to patient, as UCTD is by definition nonspecific. Symptoms typically include constitutional complaints that are common to connective tissue diseases such as fatigue, a general sense of feeling unwell, and fever.

Other symptoms associated with UCTD include:

- dry eyes

- dry mouth

- hair loss

- joint inflammation

- joint pain

- oral ulcers

- positive ANA test

- raynaud's phenomenon

- sun sensitive rash

Lung involvement, such as nonspecific interstitial pneumonia, is a possible disease complication.

Infections associated with diseases are those that are associated with possible infectious etiologies, that meet the requirements of Koch's postulates. Other methods of causation are described by the Bradford Hill criteria and Evidence-based medicine. Koch's postulates have been altered by some epidemiologists based upon sequence-based detection of distinctive pathogenic nucleic acid sequences in tissue samples. Using this method, absolute statements are not always possible regarding causation. Since this is true, higher amounts of distinctive pathogenic nucleic acid sequences would be in those exhibiting disease compared to controls since inoculating those without the pathogen is unethical. In addition, the DNA load should drop or become lower with the resolution of the disease. The distinctive pathogenic nucleic acid sequences load should also increase upon recurrence.

Other conditions are met to establish cause or association including studies in disease transmission. This means that there should be a high disease occurrence in those carrying an pathogen, evidence of a serologicalresponse to the pathogen, and the success of vaccination prevention. Direct visualization of the pathogen, the identification of different strains, immunological responses in the host, how the infection is spread and, the combination of these should all be taken into account to determine the probability that an infectious agent is the cause of the disease. A conclusive determination of a causal role of an infectious agent for in a particular disease using Koch's postulates is desired yet this might not be possible.

The leading cause of death worldwide is cardiovascular disease, but infectious diseases are the second leading cause of death worldwide and the leading cause of death in infants and children.

Chronic conditions have often been used to describe the various health related states of the human body such as syndromes, physical impairments, disabilities as well as diseases. Epidemiologists have found interest in chronic conditions due to the fact they contribute to disease, disability, and diminished physical and/or mental capacity.

For example, high blood pressure or hypertension is considered to be not only a chronic condition itself but also correlated to diseases such as heart attack or stroke. Additionally, some socioeconomic factors may be considered as a chronic condition as they lead to disability in daily life. An important one that public health officials in the social science setting have begun highlighting is chronic poverty and racism.

The list below includes these chronic conditions and diseases:

In 2015 the World Health Organisation produced a report on non-communicable diseases, citing the four major types as:

- Cancers

- Cardiovascular diseases, including cerebrovascular disease, heart failure, and ischemic cardiopathy

- Chronic respiratory diseases, such as asthma and chronic obstructive pulmonary disease (COPD)

- Diabetes mellitus

Other examples of chronic diseases and health conditions include:

- Addiction

- Alzheimer's disease

- Atrial fibrillation

- Attention deficit hyperactivity disorder

- Autoimmune diseases, such as ulcerative colitis, lupus erythematosus, Crohn's disease, coeliac disease, Hashimoto's thyroiditis, and relapsing polychondritis

- Bipolar disorder

- Blindness

- Cerebral palsy (all types)

- Chronic graft-versus-host disease (GVHD)

- Chronic hepatitis

- Chronic kidney disease

- Chronic osteoarticular diseases, such as osteoarthritis and rheumatoid arthritis

- Chronic pain syndromes, such as post-vasectomy pain syndrome and complex regional pain syndrome

- Depression

- Deafness and hearing impairment

- Eating disorders

- Ehlers–Danlos syndrome (various types)

- Endometriosis

- Epilepsy

- Fibromyalgia

- HIV/AIDS

- Huntington's disease

- Hypertension

- Lyme disease

- Multiple sclerosis

- Myalgic encephalomyelitis ( chronic fatigue syndrome)

- Narcolepsy

- Obesity

- Osteoporosis

- Parkinson's disease

- Periodontal disease

- Postural orthostatic tachycardia syndrome

- Sickle cell anemia and other hemoglobin disorders

- Sleep apnea

- Thyroid disease

- Tobacco Use and Related Conditions

An endotype is a subtype of a condition, which is defined by a distinct functional or pathobiological mechanism. This is distinct from a phenotype, which is any observable characteristic or trait of a disease, such as morphology, development, biochemical or physiological properties, or behavior, without any implication of a mechanism. It is envisaged that patients with a specific endotype present themselves within phenotypic clusters of diseases.

One example is asthma, which is considered to be a syndrome, consisting of a series of endotypes. This is related to the concept of disease entity

Collagen disease is a term previously used to describe systemic autoimmune diseases (e.g., rheumatoid arthritis, systemic lupus erythematosus, and systemic sclerosis), but now is thought to be more appropriate for diseases associated with defects in collagen, which is a component of the connective tissue.

The term "collagen disease" was coined by Dr. Alvin F. Coburn in 1932, on his quest to discover streptococcal infection as the cause for rheumatic fever.

These are also referred to as systemic autoimmune diseases. The autoimmune CTDs may have both genetic and environmental causes. Genetic factors may create a predisposition towards developing these autoimmune diseases. They are characterized as a group by the presence of spontaneous overactivity of the immune system that results in the production of extra antibodies into the circulation. The classic collagen vascular diseases have a "classic" presentation with typical findings that doctors can recognize during an examination. Each also has "classic" blood test abnormalities and abnormal antibody patterns. However, each of these diseases can evolve slowly or rapidly from very subtle abnormalities before demonstrating the classic features that help in the diagnosis. The classic collagen vascular diseases include:

- Systemic lupus erythematosus (SLE) – An inflammation of the connective tissues, SLE can afflict every organ system. It is up to nine times more common in women than men and strikes black women three times as often as white women. The condition is aggravated by sunlight.

- Rheumatoid arthritis – Rheumatoid arthritis is a systemic disorder in which immune cells attack and inflame the membrane around joints. It also can affect the heart, lungs, and eyes. Of the estimated 2.1 million Americans with rheumatoid arthritis, approximately 1.5 million (71 percent) are women.

- Scleroderma – an activation of immune cells that produces scar tissue in the skin, internal organs, and small blood vessels. It affects women three times more often than men overall, but increases to a rate 15 times greater for women during childbearing years, and appears to be more common among black women.

- Sjögren's syndrome – also called Sjögren's disease, is a chronic, slowly progressing inability to secrete saliva and tears. It can occur alone or with rheumatoid arthritis, scleroderma, or systemic lupus erythematosus. Nine out of 10 cases occur in women, most often at or around mid-life.

- Mixed connective tissue disease – Mixed connective-tissue disease (MCTD) is a disorder in which features of various connective-tissue diseases (CTDs) such as systemic lupus erythematosus (SLE); systemic sclerosis (SSc); dermatomyositis (DM); polymyositis (PM); anti-synthetase syndrome; and, occasionally, Sjögren syndrome can coexist and overlap. The course of the disease is chronic and usually milder than other CTDs. In most cases, MCTD is considered an intermediate stage of a disease that eventually becomes either SLE or Scleroderma.

- Undifferentiated connective tissue disease (UCTD) is a disease in which the body mistakenly attacks its own tissues. It is diagnosed when there is evidence of an existing autoimmune condition which does not meet the criteria for any specific autoimmune disease, such as systemic lupus erythematosus or scleroderma. Latent lupus and incomplete lupus are alternative terms that have been used to describe this condition.

- Psoriatic arthritis is also a collagen vascular disease.

A connective tissue disease is any disease that has the connective tissues of the body as a target of pathology. Connective tissue is any type of biological tissue with an extensive extracellular matrix that supports, binds together, and protects organs. These tissues form a framework, or matrix, for the body, and are composed of two major structural protein molecules: collagen and elastin. There are many different types of collagen protein in each of the body's tissues. Elastin has the capability of stretching and returning to its original length—like a spring or rubber band. Elastin is the major component of ligaments (tissues that attach bone to bone) and skin. In patients with connective tissue disease, it is common for collagen and elastin to become injured by inflammation (ICT). Many connective tissue diseases feature abnormal immune system activity with inflammation in tissues as a result of an immune system that is directed against one's own body tissues (autoimmunity).

Diseases in which inflammation or weakness of collagen tends to occur are also referred to as collagen diseases. Collagen vascular diseases can be (but are not necessarily) associated with collagen and blood vessel abnormalities and that are autoimmune in nature. See also vasculitis.

Connective tissue diseases can have strong or weak inheritance risks, and can also be caused by environmental factors.

Periodontitis as a manifestation of systemic diseases is one of the seven categories of periodontitis as defined by the American Academy of Periodontology 1999 classification system. At least 16 systemic diseases have been linked to periodontitis. These systemic diseases are associated with periodontal disease because they generally contribute to either a decreased host resistance to infections or dysfunction in the connective tissue of the gums, increasing patient susceptibility to inflammation-induced destruction.

These secondary periodontal inflammations should not be confused by other conditions in which an epidemiological association with periodontitis was revealed, but no causative connection was proved yet. Such conditions are coronary heart diseases, cerebrovascular diseases and erectile dysfunction.

Undifferentiated connective tissue disease (UCTD) is a disease in which the body mistakenly attacks its own tissues. It is diagnosed when there is evidence of an existing autoimmune condition which does not meet the criteria for any specific autoimmune disease, such as systemic lupus erythematosus or scleroderma. Latent lupus and incomplete lupus are alternative terms that have been used to describe this condition.

The term is sometimes used interchangeably with mixed connective tissue disease, an overlap syndrome. However, MCTD is thought by some researchers to be a clinically distinct entity and is strongly associated with the presence of high titers of ribonucleoprotein (RNP) antibodies.

It is estimated that up to 25 percent of people with systemic autoimmune disease could be considered to have UCTD.

A list of the more common and well-known diseases associated with infectious pathogens is provided and is not intended to be a complete listing.

By age 3 about 30% of rats have had cancer, whereas by age 85 about 30% of humans have had cancer. Humans, dogs and rabbits get Alzheimer's disease, but rodents do not. Elderly rodents typically die of cancer or kidney disease, but not of cardiovascular disease. In humans, the relative incidence of cancer increases exponentially with age for most cancers, but levels off or may even decline by age 60–75 (although colon/rectal cancer continues to increase).

People with the so-called segmental progerias are vulnerable to different sets of diseases. Those with Werner's syndrome suffer from osteoporosis, cataracts and cardiovascular disease, but not neurodegeneration or Alzheimer's disease; those with Down syndrome suffer type 2 diabetes and Alzheimer's disease, but not high blood pressure, osteoporosis or cataracts. In Bloom syndrome, those afflicted most often die of cancer.

A chronic condition is a human health condition or disease that is persistent or otherwise long-lasting in its effects or a disease that comes with time. The term "chronic" is often applied when the course of the disease lasts for more than three months. Common chronic diseases include arthritis, asthma, cancer, COPD, diabetes and some viral diseases such as hepatitis C and HIV/AIDS.

In medicine, the opposite of chronic is acute. A chronic course is further distinguished from a recurrent course; recurrent diseases relapse repeatedly, with periods of remission in between.

The non-communicable diseases are also usually lasting medical conditions but are separated by their non-infectious causes. In contrast, some chronic diseases, such as HIV/AIDS, are caused by transmissible infections.

In the United States 25% of adults have at least two chronic conditions. Chronic diseases constitute a major cause of mortality, with the World Health Organization (WHO) attributing 38 million deaths a year to non-communicable diseases.

Autoimmune diseases can be broadly divided into systemic and organ-specific or localised autoimmune disorders, depending on the principal clinico-pathologic features of each disease.

- Systemic autoimmune diseases include SLE, Sjögren's syndrome, sarcoidosis, scleroderma, rheumatoid arthritis, cryoglobulinemic vasculitis, and dermatomyositis. These conditions tend to be associated with autoantibodies to antigens which are not tissue specific. Thus although polymyositis is more or less tissue specific in presentation, it may be included in this group because the autoantigens are often ubiquitous t-RNA synthetases.

- Local syndromes which affect a specific organ or tissue:

- Endocrinologic: Diabetes mellitus type 1, Hashimoto's thyroiditis, Addison's disease

- Gastrointestinal: Coeliac disease, Crohn's disease, Pernicious anaemia

- Dermatologic: Pemphigus vulgaris, Vitiligo

- Haematologic: Autoimmune haemolytic anaemia, Idiopathic thrombocytopenic purpura

- Neurological: Multiple sclerosis, Myasthenia gravis, Encephalitis

Using the traditional “organ specific” and “non-organ specific” classification scheme, many diseases have been lumped together under the autoimmune disease umbrella. However, many chronic inflammatory human disorders lack the telltale associations of B and T cell driven immunopathology. In the last decade it has been firmly established that tissue "inflammation against self" does not necessarily rely on abnormal T and B cell responses.

This has led to the recent proposal that the spectrum of autoimmunity should be viewed along an “immunological disease continuum,” with classical autoimmune diseases at one extreme and diseases driven by the innate immune system at the other extreme. Within this scheme, the full spectrum of autoimmunity can be included. Many common human autoimmune diseases can be seen to have a substantial innate immune mediated immunopathology using this new scheme. This new classification scheme has implications for understanding disease mechanisms and for therapy development.

Autoimmunity is the system of immune responses of an organism against its own healthy cells and tissues. Any disease that results from such an aberrant immune response is termed an "autoimmune disease". Prominent examples include celiac disease, diabetes mellitus type 1, sarcoidosis, systemic lupus erythematosus (SLE), Sjögren's syndrome, eosinophilic granulomatosis with polyangiitis, Hashimoto's thyroiditis, Graves' disease, idiopathic thrombocytopenic purpura, Addison's disease, rheumatoid arthritis (RA), ankylosing spondylitis, polymyositis (PM), dermatomyositis (DM) and multiple sclerosis (MS). Autoimmune diseases are very often treated with steroids.

Symptoms start with slowly developing dysarthria (difficulty speaking) and cerebellar ataxia (unsteadiness) and then the progressive dementia becomes more evident. Loss of memory can be the first symptom of GSS. Extrapyramidal and pyramidal symptoms and signs may occur and the disease may mimic spinocerebellar ataxias in the beginning stages. Myoclonus (spasmodic muscle contraction) is less frequently seen than in Creutzfeldt–Jakob disease. Many patients also exhibit nystagmus (involuntary movement of the eyes), visual disturbances, and even blindness or deafness. The neuropathological findings of GSS include widespread deposition of amyloid plaques composed of abnormally folded prion protein.

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".

The following diseases manifest by means of neurological dysfunction: Lambert-Eaton myasthenic syndrome, paraneoplastic cerebellar degeneration, encephalomyelitis, limbic encephalitis, brainstem encephalitis, opsoclonus myoclonus ataxia syndrome, anti-NMDA receptor encephalitis, and polymyositis.

Symptoms include poor growth, loss of muscle coordination, muscle weakness, visual problems, hearing problems, learning disabilities, heart disease, liver disease, kidney disease, gastrointestinal disorders, respiratory disorders, neurological problems, autonomic dysfunction and dementia. Acquired conditions in which mitochondrial dysfunction has been involved are: diabetes, Huntington's disease, cancer, Alzheimer's disease, Parkinson's disease, bipolar disorder, schizophrenia, aging and senescence, anxiety disorders, cardiovascular disease, sarcopenia, chronic fatigue syndrome.

The body, and each mutation, is modulated by other genome variants; the mutation that in one individual may cause liver disease might in another person cause a brain disorder. The severity of the specific defect may also be great or small. Some minor defects cause only "exercise intolerance", with no serious illness or disability. Defects often affect the operation of the mitochondria and multiple tissues more severely, leading to multi-system diseases.

As a rule, mitochondrial diseases are worse when the defective mitochondria are present in the muscles, cerebrum, or nerves, because these cells use more energy than most other cells in the body.

Although mitochondrial diseases vary greatly in presentation from person to person, several major clinical categories of these conditions have been defined, based on the most common phenotypic features, symptoms, and signs associated with the particular mutations that tend to cause them.

An outstanding question and area of research is whether ATP depletion or reactive oxygen species are in fact responsible for the observed phenotypic consequences.

Cerebellar atrophy or hypoplasia has sometimes been reported to be associated.

The following diseases manifest by means of mucocutaneous dysfunction: acanthosis nigricans, dermatomyositis, Leser-Trélat sign, necrolytic migratory erythema, Sweet's syndrome, Florid cutaneous papillomatosis, pyoderma gangrenosum, and acquired generalized hypertrichosis. Mucocutaneous dysfunctions of paraneoplastic syndromes can be seen in cases of itching (hypereosinophilia), immune system depression (latent varicella-zoster virus in sensory ganglia), pancreatic tumors (leading to adipose nodular necrosis of subcutaneous tissues, flushes (prostaglandin secretions), and even dermic melanosis (cannot be eliminated via urine and results in grey to black-blueish skin tones).

Types II and III (or mixed or variant) cryoglobulinemic disease may also present with symptoms and signs of blood hyperviscosity and intravascular cryoglobulin deposition but also include those attributable to cryoglobulinemic vasculitis. "Meltzer's triad" of palpable purpura, joint pain, and generalized weakness occurs in ~33% of patients presenting with type II or type III disease. One or more skin lesions including palpable purpura, ulcers, digital gangrene, and areas of necrosis occur in 69-89% of these mixed disease cases (see attached photograph); less common findings include painful peripheral neuropathy (19-44% of cases), kidney disease (primarily membranoproliferative glomerulonephritis (30%), joint pain (28%), and, less commonly, dry eye syndrome, Raynaud phenomenon (i.e. episodic painful reductions in blood flow to the fingers and toes). While the glomerulonephritis occurring in mixed disease appears to be due to inflammatory vasculitis, the glomerulonephritis occurring in type I disease appears due to the interruption of blood flow. The hematological, infectious, and autoimmune diseases underlying type II cryoglobulinemic disease and the infectious and autoimmune diseases underlying type III cryoglobulinemic disease are also critical parts of the disease's clinical findings.

Gerstmann–Sträussler–Scheinker syndrome (GSS) is a very rare, usually familial, fatal neurodegenerative disease that affects patients from 20 to 60 years in age. Though exclusively heritable, this extremely rare disease is classified with the transmissible spongiform encephalopathies (TSE) due to the causative role played by PRNP, the human prion protein.

Familial cases are associated with autosomal-dominant inheritance.

Gerstmann–Sträussler–Scheinker disease (GSS) is an extremely rare neurogenetic brain disorder. It is always inherited and is found in only a few families all over the world (according to NINDS). The trait is an autosomal-dominant trait caused by a gene mutation. It is also in a group of hereditary prion protein diseases or also known as TSEs. Many symptoms are associated with GSS, such as progressive ataxia, pyramidal signs, and even adult-onset dementia; they progress more as the disease progresses.

Signs and symptoms due to the cryoglobulins of type I disease reflect the hyperviscosity and deposition of cryoglobulins within the blood vessels which reduce or stop blood perfusion to tissues. These events occur particularly in cases where blood cryoglobulin levels of monoclonal IgM are high in patients with IgM MGUS, smoldering Waldenström's macroglobulinemia, or Waldenström's macroglobulinemia and in uncommon cases where the levels of monoclonal IgA, IgG, free κ light chains, or free λ light chains are extremely high in patients with non-IgM MGUS, non-IgM smoldering multiple myeloma, or multiple myeloma. The interruption of blood flow to neurological tissues can cause symptoms of confusion, headache, hearing loss, and peripheral neuropathy. Interruption of blood flow to other tissues in type I disease can cause cutaneous manifestations of purpura, acrocyanosis, necrosis, ulcers, and livedo reticularis; spontaneous nose bleeds, joint pain, membranoproliferative glomerulonephritis; and cardiovascular disturbances such as shortness of breath, hypoxemia, and congestive heart failure.

For those patients with periodontitis as a manifestation of hematologic disorders, coordination with the patient's physician is instrumental in planning periodontal treatment. Therapy should be avoided during periods of exacerbation of the malignancy or during active phases of chemotherapy, and antimicrobial therapy might be considered when urgent treatment must be performed when granulocyte counts are low.