The first symptom of CJD is usually rapidly progressive dementia, leading to memory loss, personality changes, and hallucinations. Myoclonus (jerky movements) typically occurs in 90% of cases, but may be absent at initial onset. Other frequently occurring features include anxiety, depression, paranoia, obsessive-compulsive symptoms, and psychosis. This is accompanied by physical problems such as speech impairment, balance and coordination dysfunction (ataxia), changes in gait, rigid posture, and seizures. In most patients, these symptoms are accompanied by involuntary movements and the appearance of an atypical, diagnostic electroencephalogram tracing. The duration of the disease varies greatly, but sporadic (non-inherited) CJD can be fatal within months or even weeks. Most victims die six months after initial symptoms appear, often of pneumonia due to impaired coughing reflexes. About 15% of patients survive for two or more years.

The symptoms of CJD are caused by the progressive death of the brain's nerve cells, which is associated with the build-up of abnormal prion protein molecules forming amyloids. When brain tissue from a CJD patient is examined under a microscope, many tiny holes can be seen where whole areas of nerve cells have died. The word "spongiform" in "transmissible spongiform encephalopathies" refers to the sponge-like appearance of the brain tissue.

The degenerative tissue damage caused by human prion diseases (CJD, GSS, and kuru) is characterised by four features: spongiform change, neuronal loss, astrocytosis, and amyloid plaque formation. These features are shared with prion diseases in animals, and the recognition of these similarities prompted the first attempts to transmit a human prion disease (kuru) to a primate in 1966, followed by CJD in 1968 and GSS in 1981. These neuropathological features have formed the basis of the histological diagnosis of human prion diseases for many years, although it was recognized that these changes are enormously variable both from case to case and within the central nervous system in individual cases.

The clinical signs in humans vary, but commonly include personality changes, psychiatric problems such as depression, lack of coordination, and/or an unsteady gait (ataxia). Patients also may experience involuntary jerking movements called myoclonus, unusual sensations, insomnia, confusion, or memory problems. In the later stages of the disease, patients have severe mental impairment (dementia) and lose the ability to move or speak.

Early neuropathological reports on human prion diseases suffered from a confusion of nomenclature, in which the significance of the diagnostic feature of spongiform change was occasionally overlooked. The subsequent demonstration that human prion diseases were transmissible reinforced the importance of spongiform change as a diagnostic feature, reflected in the use of the term "spongiform encephalopathy" for this group of disorders.

Prions appear to be most infectious when in direct contact with affected tissues. For example, Creutzfeldt–Jakob disease has been transmitted to patients taking injections of growth hormone harvested from human pituitary glands, from cadaver dura allografts and from instruments used for brain surgery (Brown, 2000) (prions can survive the "autoclave" sterilization process used for most surgical instruments). It is also believed that dietary consumption of affected animals can cause prions to accumulate slowly, especially when cannibalism or similar practices allow the proteins to accumulate over more than one generation. An example is kuru, which reached epidemic proportions in the mid-20th century in the Fore people of Papua New Guinea, who used to consume their dead as a funerary ritual. Laws in developed countries now ban the use of rendered ruminant proteins in ruminant feed as a precaution against the spread of prion infection in cattle and other ruminants.

There exist evidence that prion diseases may be transmissible by the airborne route.

Note that not all encephalopathies are caused by prions, as in the cases of PML (caused by the JC virus), CADASIL (caused by abnormal NOTCH3 protein activity), and Krabbe disease (caused by a deficiency of the enzyme galactosylceramidase). Progressive Spongiform Leukoencephalopathy (PSL)—which is a spongiform encephalopathy—is also probably not caused by a prion, although the adulterant that causes it among heroin smokers has not yet been identified. This, combined with the highly variable nature of prion disease pathology, is why a prion disease cannot be diagnosed based solely on a patient's symptoms.

Kuru, a transmissible spongiform encephalopathy, is a disease of the nervous system that causes physiological and neurological effects which ultimately lead to death. It is characterized by progressive cerebellar ataxia, or loss of coordination and control over muscle movements.

The preclinical or asymptomatic phase, also called the incubation period, averages between 10–13 years, but can be as short as 5 and has been estimated to last as long as 50 years or more after initial exposure. The youngest individual recorded to have kuru was 12 years old.

The clinical stage, which begins at the first onset of symptoms, lasts an average of 12 months. The clinical progression of kuru is divided into three specific stages, the ambulant, sedentary and terminal stages. While there is some variation in these stages between individuals, they are highly conserved among the affected population. Before the onset of clinical symptoms, an individual can also present with prodromal symptoms including headache and joint pain in the legs.

In the first (ambulant) stage, the infected individual may exhibit unsteady stance and gait, decreased muscle control, tremors, difficulty pronouncing words (dysarthria), and titubation. This stage is named the ambulant because the individual is still able to walk around despite symptoms.

In the second (sedentary) stage, the infected individual is incapable of walking without support and suffers ataxia and severe tremors. Furthermore, the individual shows signs of emotional instability and depression, yet exhibits uncontrolled and sporadic laughter. Despite the other neurological symptoms, tendon reflexes are still intact at this stage of the disease.

In the third and final (terminal) stage, the infected individual’s existing symptoms, like ataxia, progress to the point where they are no longer capable of sitting without support. New symptoms also emerge: the individual develops dysphagia, or difficulty swallowing, which can lead to severe malnutrition. They may also become incontinent, lose the ability to speak and become unresponsive to their surroundings, despite maintaining consciousness. Towards the end of the terminal stage patients often develop chronic ulcerated wounds that can be easily infected. An infected person usually dies within three months to two years after the first terminal stage symptoms, often because of pneumonia or infection.

Transmissible spongiform encephalopathies (TSEs), also known as prion diseases, are a group of progressive, invariably fatal, conditions that affect the brain (encephalopathies) and nervous system of many animals, including humans. According to the most widespread hypothesis, they are transmitted by prions, though some other data suggest an involvement of a "Spiroplasma" infection. Mental and physical abilities deteriorate and myriad tiny holes appear in the cortex causing it to appear like a sponge (hence spongiform) when brain tissue obtained at autopsy is examined under a microscope. The disorders cause impairment of brain function, including memory changes, personality changes and problems with movement that worsen chronically.

Prion diseases of humans include Creutzfeldt–Jakob disease—which has four main forms, the sporadic (sCJD), the hereditary/familiar (fCJD), the iatrogenic (iCJD) and the variant form (vCJD)—Gerstmann–Sträussler–Scheinker syndrome, fatal familial insomnia, kuru, and the recently discovered variably protease-sensitive prionopathy. These conditions form a spectrum of diseases with overlapping signs and symptoms. TSEs in non-human mammals include scrapie in sheep, bovine spongiform encephalopathy (BSE)—popularly known as 'mad cow's disease'—in cattle and chronic wasting disease (CWD) in deer and elk. The variant form of Creutzfeldt–Jakob disease is caused by exposure to bovine spongiform encephalopathy prions.

Unlike other kinds of infectious disease, which are spread by agents with a DNA or RNA genome (such as virus or bacteria), the infectious agent in TSEs is believed to be a prion, thus being composed solely of protein material. Misshapen prion proteins carry the disease between individuals and cause deterioration of the brain. TSEs are unique diseases in that their aetiology may be genetic, sporadic, or infectious via ingestion of infected foodstuffs and via iatrogenic means (e.g., blood transfusion). Most TSEs are sporadic and occur in an animal with no prion protein mutation. Inherited TSE occurs in animals carrying a rare mutant prion allele, which expresses prion proteins that contort by themselves into the disease-causing conformation. Transmission occurs when healthy animals consume tainted tissues from others with the disease. In the 1980s and 1990s, bovine spongiform encephalopathy (BSE) spread in cattle in an epidemic fashion. This occurred because cattle were fed the processed remains of other cattle, a practice now banned in many countries. In turn, consumption (by humans) of bovine-derived foodstuff which contained prion-contaminated tissues resulted in an outbreak of the variant form of Creutzfeldt–Jakob disease in the 1990s and 2000s.

Prions cannot be transmitted through the air or through touching or most other forms of casual contact. However, they may be transmitted through contact with infected tissue, body fluids, or contaminated medical instruments. Normal sterilization procedures such as boiling or irradiating materials fail to render prions non-infective.

Creutzfeldt–Jakob disease (CJD) is a universally fatal brain disorder. Early symptoms include memory problems, behavioral changes, poor coordination, and visual disturbances. Later dementia, involuntary movements, blindness, weakness, and coma occur. About 90% of people die within a year of diagnosis.

CJD is believed to be caused by a protein known as a prion. Infectious prions are misfolded proteins that can cause normally folded proteins to become misfolded. Most cases occur spontaneously, while about 7.5% of cases are inherited from a person's parents in an autosomal dominant manner. Exposure to brain or spinal tissue from an infected person may also result in spread. There is no evidence that it can spread between people via normal contact or blood transfusions. Diagnosis involves ruling out other potential causes. An electroencephalogram, spinal tap, or magnetic resonance imaging may support the diagnosis.

There is no specific treatment. Opioids may be used to help with pain, while clonazepam or sodium valproate may help with involuntary movements. CJD affects about one per million people per year. Onset is typically around 60 years of age. The condition was first described in 1920. It is classified as a type of transmissible spongiform encephalopathy. CJD is different from bovine spongiform encephalopathy (mad cow disease) and variant Creutzfeldt–Jakob disease (vCJD).

Symptoms are not seen immediately in cattle due to the diseases’ extremely long incubation period. Some cattle have been observed to have an abnormal gait, changes in behavior, tremors and hyper-responsiveness to certain stimuli. Hindlimb ataxia affects the animal’s gait and occurs when muscle control is lost. This results in poor balance and coordination. Behavioural changes may include aggression, anxiety relating to certain situations, nervousness, frenzy or an overall change in temperament. Some rare but previously observed symptoms also include persistent pacing, rubbing or licking. Additionally, nonspecific symptoms have also been observed which include weight loss, decreased milk production, lameness, ear infections and teeth grinding due to pain. Some animals may show a combination of these symptoms, while others may only be observed demonstrating one of the many reported. Once clinical symptoms arise, they typically get worse over the upcoming weeks and months, eventually leading to recumbency, coma and death.

BSE is a transmissible disease that primarily affects the central nervous system; it is a form of transmissible spongiform encephalopathy, like Creutzfeldt–Jakob disease and kuru in humans and scrapie in sheep, and chronic wasting disease in cervids.

A slow virus is a virus, or a viruslike agent, etiologically associated with a disease, having a long incubation period of months to years and then a gradual onset of symptoms which progress slowly but irreversibly and terminate in a severe compromised state or, more commonly, death.

A slow virus disease is a disease that, after an extended period of latency, follows a slow, progressive course spanning months to years, frequently involving the central nervous system and ultimately leading to death. Examples include the Visna-Maedi virus, in the genus Lentivirus (family Retroviridae), that causes encephalitis and chronic pneumonitis in sheep, and subacute sclerosing panencephalitis which is apparently caused by the measles virus, as well as Paget's Disease of Bone (Osteitis Deformans) which is associated with paramyxoviridae, especially RSV and Rubeola (Measles).

Variant Creutzfeldt–Jakob disease (vCJD) or new variant Creutzfeldt–Jakob disease (nvCJD) is a transmissible spongiform encephalopathy which was identified in 1996 by the National CJD Surveillance Unit in Edinburgh, Scotland. It is always fatal and is caused by prions, which are mis-folded proteins. Over 170 cases of vCJD have been recorded in the United Kingdom, and around 30 cases in the rest of the world. The fact that the epidemiology of the disease coincided with an epidemic of bovine spongiform encephalopathy led to the hypothesis that consumption of BSE-infected beef caused the disease. It is a different disease from Sporadic and Familial Creutzfeldt–Jakob disease, though it is believed to be caused by the same pathogenic agent, a mis-folded protein, known as a prion.

Despite the consumption of contaminated beef in the UK being reckoned to be quite high, vCJD has infected a comparatively small cohort of people. One explanation for this can be found in the genetics of patients with the disease. The human PRNP protein which is subverted in prion disease can occur with either methionine or valine at amino acid 129, without any apparent difference in normal function. Of the overall Caucasian population, about 40% have two methionine-containing alleles, 10% have two valine-containing alleles, and the other 50% are heterozygous at this position. Only a single vCJD patient tested was found to be heterozygous; most of those affected had two copies of the methionine-containing form. Additionally, for unknown reasons, those affected are generally under the age of 40. It is not yet known whether those unaffected are actually immune or only have a longer incubation period until symptoms appear.

Kuru is a very rare, incurable neurodegenerative disorder that was formerly common among the Fore people of Papua New Guinea. Kuru is caused by the transmission of abnormally folded prion proteins, which leads to symptoms such as tremors, loss of coordination, and neurodegeneration.

The term kuru derives from the Fore word kuria or guria ("to shake"), due to the body tremors that are a classic symptom of the disease and kúru itself means "trembling". It is also known as the "laughing sickness" due to the pathologic bursts of laughter which are a symptom of the disease. It is now widely accepted that kuru was transmitted among members of the Fore tribe of Papua New Guinea via funerary cannibalism. Deceased family members were traditionally cooked and eaten, which was thought to help free the spirit of the dead. Females and children usually consumed the brain, the organ in which infectious prions were most concentrated, thus allowing for transmission of kuru. The disease was therefore more prevalent among women and children.

While the Fore people stopped eating human meat in the early 1960's, when it was first speculated to be transmitted via endocannibalism, the disease lingered due to kuru’s long incubation period of anywhere from 10 to over 50 years. The epidemic declined sharply after discarding cannibalism, from 200 deaths per year in 1957 to 1 or no deaths annually in 2005, with sources disagreeing on whether the last known kuru victim died in 2005 or 2009.

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.

Every infectious agent is different, but in general, slow viruses:

Additionally, the immune system seems to plays a limited role, or no role, in protection from these slow viruses. This may be in part because the host has acclimated to the virus, or more likely because the host must be immunocompromised in order for many of these slow virus infections to emerge, so the immune system is at a disadvantage from the start.

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.

Variably protease-sensitive prionopathy (VPSPr) (formerly known as Protease Sensitive Prionopathy) is a sporadic prion protein disease identified in 2008 and first described in 2010 by Zou W.Q. and coworkers from the United States National Prion Disease Pathology Surveillance Center.

VPSPr is very rare, occurring in just 2 or 3 out of every 100 million people. (Nine cases had been identified in the UK by 2013.) It has similarities to Creutzfeldt–Jakob disease, but clinical manifestations differ somewhat, and the abnormal prion protein (PrP) is less resistant to digestion by proteases; some variants are more sensitve to proteases than others, hence the name: variably protease-sensitive.

Patients present with psychiatric symptoms, speech deficits (aphasia and/or dysarthria), and cognitive impairment. Ataxia and parkinsonism can develop. Average age at onset is 70 years, and duration of survival is 24 months. About 40% of patients have a family history of dementia.

Diagnosis is difficult. MRI, EEG, and tests for 14-3-3 protein and tau protein are usually not helpful, and no mutations have been observed in the coding region of the PrP gene.

There are examples of slowly and rapidly progressive diseases affecting all organ systems and parts of the body. The following are some examples of rapidly and slowly progressive diseases affecting various organ systems:

- Brain: Creutzfeldt–Jakob disease progresses rapidly compared to Alzheimer's disease.

- Eyes: Cataracts can be static or slowly progressive. Macular degeneration is slowly progressive, while retinal detachment is rapidly progressive.

- Lungs: Emphysema due to alpha-1 antitrypsin deficiency is a slowly progressive pulmonary disease.

- Kidneys: Goodpasture's syndrome is a rapidly progressive glomerulonephritis, while diabetic glomerulosclerosis is slowly progressive.

- Pancreas: Type 1 diabetes mellitus involves rapidly progressive loss of insulin secretory capacity compared to type 2 diabetes mellitus, in which the loss of insulin secretion is slowly progressive over many years. MODY 2, due to "GCK" mutation, is a relatively static form of reduced insulin secretion.

- Joints: Both osteoarthritis and rheumatoid arthritis are slowly progressive forms of arthritis.

- Nerves: Essential tremor is a slowly progressive neurological disorder which is usually genetically passed down.

Signs and symptoms of GM2-gangliosidosis, AB variant are identical with those of infantile Tay-Sachs disease, except that enzyme assay testing shows normal levels of hexosaminidase A. Infantile Sandhoff disease has similar symptoms and prognosis, except that there is deficiency of both hexosaminidase A and hexosaminidase B. Infants with this disorder typically appear normal until the age of 3 to 6 months, when development slows and muscles used for movement weaken. Affected infants lose motor skills such as turning over, sitting, and crawling. As the disease progresses, infants develop seizures, vision and hearing loss, mental retardation, and paralysis.

An ophthalmological abnormality called a cherry-red spot, which can be identified with an eye examination, is characteristic of this disorder. This cherry-red spot is the same finding that Warren Tay first reported in 1881, when he identified a case of Tay-Sachs disease, and it has the same etiology.

The prognosis for AB variant is the same as for infantile Tay-Sachs disease. Children with AB variant die in infancy or early childhood.

Progressive disease or progressive illness is a disease or physical ailment whose course in most cases is the worsening, growth, or spread of the disease. This may happen until death, serious debility, or organ failure occurs. Some progressive diseases can be halted and reversed by treatment. Many can be slowed by medical therapy. Some cannot be altered by current treatments.

Though the time distinctions are imprecise, diseases can be "rapidly progressive" (typically days to weeks) or "slowly progressive" (months to years). Virtually all slowly progressive diseases are also chronic diseases in terms of time course; many of these are also referred to as degenerative diseases. Not all chronic diseases are progressive: a chronic, non-progressive disease may be referred to as a "static" condition.

"Progressive disease" can also be a clinical endpoint i.e. an endpoint in a clinical trial.

In medicine, proteopathy (Proteo- ["pref". protein]; -pathy ["suff". disease]; proteopathies "pl".; proteopathic "adj".) refers to a class of diseases in which certain proteins become structurally abnormal, and thereby disrupt the function of cells, tissues and organs of the body. Often the proteins fail to fold into their normal configuration; in this misfolded state, the proteins can become toxic in some way (a gain of toxic function) or they can lose their normal function. The proteopathies (also known as proteinopathies, protein conformational disorders, or protein misfolding diseases) include such diseases as Creutzfeldt–Jakob disease and other prion diseases, Alzheimer's disease, Parkinson's disease, amyloidosis, and a wide range of other disorders (see List of Proteopathies).

The concept of proteopathy can trace its origins to the mid-19th century, when, in 1854, Rudolf Virchow coined the term amyloid ("starch-like") to describe a substance in cerebral corpora amylacea that exhibited a chemical reaction resembling that of cellulose. In 1859, Friedreich and Kekulé demonstrated that, rather than consisting of cellulose, "amyloid" actually is rich in protein. Subsequent research has shown that many different proteins can form amyloid, and that all amyloids have in common birefringence in cross-polarized light after staining with the dye Congo Red, as well as a fibrillar ultrastructure when viewed with an electron microscope. However, some proteinaceous lesions lack birefringence and contain few or no classical amyloid fibrils, such as the diffuse deposits of Aβ protein in the brains of Alzheimer patients. Furthermore, evidence has emerged that small, non-fibrillar protein aggregates known as oligomers are toxic to the cells of an affected organ, and that amyloidogenic proteins in their fibrillar form may be relatively benign.

GM2-gangliosidosis, AB variant is a rare, autosomal recessive metabolic disorder that causes progressive destruction of nerve cells in the brain and spinal cord. It has a similar pathology to Sandhoff disease and Tay-Sachs disease. The three diseases are classified together as the GM2 gangliosidoses, because each disease represents a distinct molecular point of failure in the activation of the same enzyme, beta-hexosaminidase. AB variant is caused by a failure in the gene that makes an enzyme cofactor for beta-hexosaminidase, called the GM2 activator.

The age of onset is variable, ranging from 18 to 60, with an average of 50. The disease can be detected prior to onset by genetic testing. Death usually occurs between seven and thirty-six months from onset. The presentation of the disease varies considerably from person to person, even among patients from within the same family.

The disease has four stages:

1. The person has increasing insomnia, resulting in panic attacks, paranoia, and phobias. This stage lasts for about four months.

2. Hallucinations and panic attacks become noticeable, continuing for about five months.

3. Complete inability to sleep is followed by rapid loss of weight. This lasts for about three months.

4. Dementia, during which the patient becomes unresponsive or mute over the course of six months. This is the final progression of the disease, after which death follows.

Other symptoms include profuse sweating, pinpoint pupils, the sudden entrance into menopause for women and impotence for men, neck stiffness, and elevation of blood pressure and heart rate. Constipation is common as well. As the disease progresses, the patient will become stuck in a state of pre-sleep limbo, or hypnagogia, which is the state just before sleep in healthy individuals. During these stages, it is common for patients to repeatedly move their limbs as if dreaming.

The first reported case in the Netherlands was of a 57-year-old man of Egyptian descent. The man came in with symptoms of double vision and progressive memory loss, and his family also noted he had recently become disoriented, paranoid, and confused. While he tended to fall asleep during random daily activities, he experienced vivid dreams and random muscular jerks during normal slow wave sleep. After four months of these symptoms, he started having convulsions in the hands, trunk, and lower limbs while awake. The patient died at 58 (seven months after the onset of symptoms). An autopsy was completed which revealed mild atrophy of the frontal cortex and moderate atrophy of the thalamus. The atrophy of the thalamus is one of the most common signs of fatal familial insomnia.

Progressive inflammatory neuropathy (PIN) is a disease that was identified in a report, released on January 31, 2008, by the Centers for Disease Control and Prevention. The first known outbreak of this neuropathy occurred in southeastern Minnesota in the United States. The disease was reported among pig slaughterhouse workers who appeared at various care facilities in the area reporting similar neurological symptoms. The disease was later identified at pork processing plants in Indiana and Nebraska as well. The condition is characterized by acute paralysis, pain, fatigue, numbness, and weakness, especially in extremities. It was initially believed that workers might have contracted the disease through inhaling aerosols from pig brains blown through a compressed-air hose and that this exposure to pig neural tissue induced an autoimmune response that might have produced their mysterious peripheral neuropathy. These suspicions were confirmed in reports and investigations conducted at the Mayo Clinic in Rochester, Minnesota.

Inflammatory demyelinating diseases (IDDs), sometimes called Idiopathic (IIDDs) because the unknown etiology of some of them, and sometimes known as borderline forms of multiple sclerosis, is a collection of multiple sclerosis variants, sometimes considered different diseases, but considered by others to form a spectrum differing only in terms of chronicity, severity, and clinical course.

Multiple Sclerosis for some people is a syndrome more than a single disease. It can be considered among the acquired demyelinating syndromes with a multiphasic instead of monophasic behaviour. Multiple sclerosis also has a prodromal stage in which an unknown underlying condition, able to damage the brain, is present, but no lesion has still developed.

Normal sleep has different stages that together last 90 to 100 minutes:

- Non REM Stages 1 and 2: Light sleep NREM-sleep

- Non REM Stage 3 (previously 3 and 4): Deep slow wave sleep (SWS)

- REM-sleep when memorable dreams occur

FFI patients cannot go past stage 1 and thus their brains are not getting the rest they need to revive, as most reviving and repairing processes of the body are believed to happen during these deeper sleep stages.

The list of these diseases depends of the author, but usually are included:

- multiple sclerosis, normally defined by the dissemination in time and space of demyelinating lesions, with two (or sometimes three) clinical presentations:

- Relapsing-Onset multiple sclerosis, the most known and extended variant, normally consisting of two distinct clinical phases (Remitent-Recidivant, RRMS, and Secondary Progressive, SPMS)

- Progressive-Onset MS, most known as Primary progressive MS including a special genetic variant named rapidly progressive multiple sclerosis.

- Optic-spinal MS, or opticospinal, clinical and pathological variant of multiple sclerosis which often include visual symptoms and have a more severe course than typical MS. Though multiple scars (scleroses) are present in CNS, and they comply with the dissemination criteria, and sometimes is classified as clinically definite multiple sclerosis, currently is considered outside the scope of Multiple Sclerosis and inside the scope of Devic's disease, though it is uncertain if this applies to all cases. Also a variant affecting mainly the spinal cord and the cortex has been proposed

- Neuromyelitis optica (NMO), and its associated "spectrum of disorders" (NMOSD), currently considered a common syndrome for at least three separated diseases:, mainly produced by AQP4 autoimmune channelopathy, though other variants exists, some with anti-MOG and some others idiopathic. Some researchers think that there could exist an overlapping between Anti-NMDA receptor encephalitis cases and neuromyelitis optica or acute disseminated encephalomyelitis.

- Anti-MOG associated spectrum, often clinically presented as an anti-MOG autoimmune encephalomyelitis, but can also appear as negative NMO or atypical multiple sclerosis

- CRION (Chronic relapsing inflammatory optic neuritis): A distinct clinical entity from other inflammatory demyelinating diseases including multiple sclerosis (MS), neuromyelitis optica-immunoglobulin G (NMO-IgG) spectrum disease, and idiopathic relapsing optic neuritis.

- Acute disseminated encephalomyelitis or ADEM, a closely related disorder in which a known virus or vaccine triggers autoimmunity against myelin.

- Acute hemorrhagic leukoencephalitis, possibly a variant of Acute disseminated encephalomyelitis

- Balo concentric sclerosis, an unusual presentation of plaques forming concentrenic circles, which can sometimes get better spontaneously.

- Schilder disease or diffuse myelinoclastic sclerosis: is a rare disease that presents clinically as a pseudotumoural demyelinating lesion; and is more common in children.

- Marburg multiple sclerosis, an aggressive form, also known as malignant, fulminant or acute MS.

- Tumefactive multiple sclerosis: lesions whose size is more than 2 cm, with mass effect, oedema and/or ring enhancement

- Solitary sclerosis: This variant has been recently proposed (2012) by Mayo Clinic researches. though it was also reported by other groups more or less at the same time. It is defined as isolated demyelinating lesions which produce a progressive myelopathy similar to primary progressive MS, and is currently considered a synonym for tumefactive multiple sclerosis.

Some inflammatory conditions are associated with the presence of scleroses in the CNS. Optic neuritis (monophasic and recurrent) and Transverse myelitis (monophasic and recurrent)

As MS is an active field for research, the list is not closed or definitive. For example, some diseases like Susac's syndrome (MS has an important vascular component), leukoaraiosis, myalgic encephalomyelitis (aka chronic fatigue syndrome) or autoimmune variants of peripheral neuropathies like Guillain–Barré syndrome or progressive inflammatory neuropathy could be included assuming the autoimmune model. Also Leukodystrophy (which see) and its sub-conditions: Adrenoleukodystrophy and Adrenomyeloneuropathy could be in the list. Venous induced demyelination has also been proposed as a hypothetical MS variant produced by CCSVI.

Recent research has identified some possible new variants, like the possibility to separate primary progressive MS, PPMS, after recent findings seem to point that it is pathologically a very different disease.

Also an OPA1 variant and aKIR4.1 multiple sclerosis variant was reported in 2012 and later reported again, which could be considered a different disease (as Devic disease did before), and can represent up to a 47% of the MS cases. Finally, there exist some reports of an aquaporine-related multiple sclerosis, related to vegetal aquaporine proteins.

Posterior cortical atrophy (PCA), also called Benson's syndrome, is a form of dementia which is usually considered an atypical variant of Alzheimer's disease (AD). The disease causes atrophy of the posterior part of the cerebral cortex, resulting in the progressive disruption of complex visual processing. PCA was first described by D. Frank Benson in 1988.

In rare cases, PCA can be caused by dementia with Lewy bodies and Creutzfeldt–Jakob disease.

PCA usually affects people at an earlier age than typical cases of Alzheimer's disease, with initial symptoms often experienced in people in their mid-fifties or early sixties. This was the case with writer Terry Pratchett (1948-2015), who went public in 2007 about being diagnosed with PCA. In "The Mind's Eye", neurologist Oliver Sacks examines the case of concert pianist Lilian Kallir (1931–2004), who suffered from PCA.