The first symptoms of Guillain–Barré syndrome are numbness, tingling, and pain, alone or in combination. This is followed by weakness of the legs and arms that affects both sides equally and worsens over time. The weakness can take half a day to over two weeks to reach maximum severity, and then becomes steady. In one in five people, the weakness continues to progress for as long as four weeks. The muscles of the neck may also be affected, and about half experience involvement of the cranial nerves which supply the head and face; this may lead to weakness of the muscles of the face, swallowing difficulties and sometimes weakness of the eye muscles. In 8%, the weakness affects only the legs (paraplegia or paraparesis). Involvement of the muscles that control the bladder and anus is unusual. In total, about a third of people with Guillain–Barré syndrome continue to be able to walk. Once the weakness has stopped progressing, it persists at a stable level ("plateau phase") before improvement occurs. The plateau phase can take between two days and six months, but the most common duration is a week. Pain-related symptoms affect more than half, and include back pain, painful tingling, muscle pain and pain in the head and neck relating to irritation of the lining of the brain.

Many people with Guillain–Barré syndrome have experienced the signs and symptoms of an infection in the 3–6 weeks prior to the onset of the neurological symptoms. This may consist of upper respiratory tract infection (rhinitis, sore throat) or diarrhea.

In children, particularly those younger than six years old, the diagnosis can be difficult and the condition is often initially mistaken (sometimes for up to two weeks) for other causes of pains and difficulty walking, such as viral infections, or bone and joint problems.

On neurological examination, characteristic features are the reduced power and reduced or absent tendon reflexes (hypo- or areflexia, respectively). However, a small proportion has normal reflexes in affected limbs before developing areflexia, and some may have exaggerated reflexes. In the "Miller Fisher variant" subtype of Guillain–Barré syndrome (see below), a triad of weakness of the eye muscles, abnormalities in coordination, as well as absent reflexes can be found. The level of consciousness is normally unaffected in Guillain–Barré syndrome, but the Bickerstaff brainstem encephalitis subtype may feature drowsiness, sleepiness, or coma.

A quarter of all people with Guillain–Barré syndrome develop weakness of the breathing muscles leading to respiratory failure, the inability to breathe adequately to maintain healthy levels of oxygen and/or carbon dioxide in the blood. This life-threatening scenario is complicated by other medical problems such as pneumonia, severe infections, blood clots in the lungs and bleeding in the digestive tract in 60% of those who require artificial ventilation.

In order to diagnose Bickerstaff brainstem encephalitis, ataxia and ophthalmoplegia must be present. These are also diagnostic features of Miller Fisher syndrome, and so Bickerstaff's is only diagnosed if other features are present which exclude Miller Fisher syndrome. These may include drowsiness, coma or hyperreflexia. When the condition is defined in this way, a number of other features are commonly but not always found: among these are weakness of the limbs, the face, and/or the bulbar muscles; abnormalities of the pupils; and absent reflexes.

Like some other autoimmune diseases, the condition usually follows a minor infection, such as a respiratory tract infection or gastroenteritis.

Bickerstaff brainstem encephalitis is a rare inflammatory disorder of the central nervous system, first described by Edwin Bickerstaff in 1951. It may also affect the peripheral nervous system, and has features in common with both Miller Fisher syndrome and Guillain–Barré syndrome.

Acute cerebellar ataxia usually follows 2–3 weeks after an infection. Onset is abrupt. Vomiting may be present at the onset but fever and nuchal rigidity characterestically are absent. Horizontal nystagmus is present is approximately 50% of cases.

- Truncal ataxia with deterioration of gait

- Slurred speech and nystagmus

- Afebrile

Acute cerebellar ataxia of childhood is a childhood condition characterized by an unsteady gait, most likely secondary to an autoimmune of postinfectious cause, drug induced or paraneoplastic. Most common virus causing acute cerebellar ataxia are Chickenpox virus and Epstein Barr Virus. It is a diagnosis of exclusion.

Paresthesia is an abnormal sensation such as tingling, tickling, pricking, numbness or burning of a person's skin with no apparent physical cause. The manifestation of a paresthesia may be transient or chronic, and may have any of dozens of possible underlying causes.

The most familiar kind of paresthesia is the sensation known as "pins and needles" or of a limb "falling asleep". A less well-known and uncommon but important paresthesia is formication, the sensation of bugs crawling underneath the skin.

Paresthesias of the hands, feet, legs and arms are common, transient symptoms. The briefest, electric shock type of paresthesia can be caused by tweaking the ulnar nerve near the elbow. Similar brief shocks can be experienced when any other nerve is tweaked (a tweaked neck nerve may cause a brief shock-like paresthesia toward the scalp). In the older age group, spinal column irregularities may tweak the spinal cord briefly when the head or back is turned, flexed, or extended into brief uncommon positions (Lhermitte's sign). The most common, everyday cause is temporary restriction of nerve impulses to an area of nerves, commonly caused by leaning or resting on parts of the body such as the legs (often followed by a pins and needles tingling sensation). Other causes include conditions such as hyperventilation syndrome and panic attacks. A cold sore outside the mouth (not a canker sore inside the mouth) can be preceded by tingling because a cold sore is caused by herpes simplex virus. The Varicella zoster virus (Shingles) also notably may cause recurring pain and tingling in skin or tissue along the distribution path of that nerve (most commonly in skin, along a dermatome pattern, but sometimes feeling like headache, chest or abdominal pain, or pelvic pain).

Other common examples occur when sustained pressure has been applied over a nerve, inhibiting or stimulating its function. Removing the pressure typically results in gradual relief of these paresthesias. Most pressure-induced paraesthesia results from awkward posture, such as engaging in cross-legged sitting for prolonged periods of time.

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.

Over 40 laboratory tests were initially conducted to rule out various pathogens and environmental toxins. These tests were used to try to identify potential viruses carried by humans, pigs, or both, including rotoviruses, adenoviruses, hepatitis A, and hepatitis E. They also tried to identify bacteria such as salmonella and escherichia coli (e. coli), and parasites such as Giardia and cryptosporidium that could be causing the symptoms. All were ruled out.

Neurodegenerative diseases were considered specifically because of the similarity of symptoms and animal involvement thus included investigation of prion associated diseases such as bovine spongiform encephalopathy (BSE), chronic wasting disease (CWD), and variant Creutzfeldt–Jakob disease (vCJD). These all have highly transmissible pathogenic agents that induce brain damage. Since no pathogenic agent had been found, these diseases were ruled out as being related.

Next two very similar neuropathies were ruled out. Guillain–Barré syndrome (GBS) induces an acute autoimmune response which affects the Schwann cells in the peripheral nervous system. GBS is usually triggered by an infection that causes weakness and tingling that may lead to muscle loss. This condition may be life-threatening if muscle atrophy ascends to affect the pulmonary or cardiac systems. So far, no infectious agents have been found that relate to the current disease, progressive infammatory neuropathy. They looked at chronic inflammatory demyelinating polyneuropathy (CIDP) which is characterized by progressive weakness and sensory impairment in the arms and legs. Damage occurs to the myelin sheath in the peripheral nervous system. As doctors at the Mayo Clinic were beginning to note, the problem they were seeing in progressive inflammatory neuropathy was occurring in the spinal nerve roots.

As of 2007, fewer than 500 Yakut individuals have been infected with VE. Viliuisk Encephalomyelitis is classified as a progressive neurological disorder that ultimately ends in the death of the infected individual. The disease has three distinguishable phases: The acute form, the progressive form, and the chronic form.

The acute form is the most rapid and most violent of all the stages. It begins with the characteristic rigidity of the muscles, accompanied by slurred speech, severe headaches, and exaggeration of cold-like symptoms. Patients usually die within weeks of the initial symptoms. Routine post-mortem examinations yield: severe inflammation of the brain lining, clusters of dead cells and tissue, and largely increased amounts of macrophages and lymphocytes.

The progressive form is the most common case. Patients initially experience acute-like symptoms which are not as severe, and subside within a few weeks. Following the sub-acute phase, the patients experience a few mild symptoms including some behavioral changes, incoordination, and difficulty in speech. Eventually the disease developed fully and those infected were stricken with the characteristic symptoms of rigidity, slurred speech, and deterioration of cognitive functions. Ultimately, brain function depreciates rapidly resulting in death.

Many patients who undergo the chronic form claim never to have had an acute attack. These patients endure varying measures of impairment and suffer mental deterioration for the remainder of their lives. Usually they live to be very old and succumb to other diseases.

In almost all cases there are changes characteristic of VE. Early onset shows an increased number of lymphocytes and increased protein concentration — which reduces over many years. These factors help neurologists determine the form of VE based on progression. The trademark changes in the brain include: thickened inflamed meninges, necrotic cortical lesions, increased number of lymphocytes, and neuronal death.

Not all acute diseases or injuries are severe, and vice versa. For example, a mild stubbed toe is an acute injury. Similarly, many acute upper respiratory infections and acute gastroenteritis cases in adults are mild and usually resolve within a few days or weeks.

The term "acute" is also included in the definition of several diseases, such as severe acute respiratory syndrome, acute leukemia, acute myocardial infarction, and acute hepatitis. This is often to distinguish diseases from their chronic forms, such as chronic leukemia, or to highlight the sudden onset of the disease, such as acute myocardial infarct.

In medicine, describing a disease as acute denotes that it is of short and, as a corollary of that, of recent . The quantitation of how much time constitutes "short" and "recent" varies by disease and by context, but the core denotation of "acute" is always qualitatively in contrast with "chronic", which denotes long-lasting disease (for example, in acute leukemia and chronic leukemia). In addition, "acute" also often connotes two other meanings: onset and , such as in acute myocardial infarction (EMI), where suddenness and severity are both established aspects of the meaning. It thus often connotes that the condition is fulminant (as in the EMI example), but not always (as in acute rhinitis, which is usually synonymous with the common cold). The one thing that acute MI and acute rhinitis have in common is that they are not chronic. They can happen again (as in recurrent pneumonia, that is, multiple acute pneumonia episodes), but they are not the same ongoing for months or years (unlike chronic obstructive pulmonary disease, which is).

A noncount sense of "acute disease" refers to the acute phase, that is, a short course, of any disease entity. For example, in an article on ulcerative enteritis in poultry, the author says, "in acute disease there may be increased mortality without any obvious signs", referring to the acute form or phase of ulcerative enteritis.

AEN has never been recorded as a one symptom disorder, but instead present by multiple symptoms. The symptoms vary from the severity of the disorder. The most classic sign of AEN is the dark pigmentation of esophageal mucosa in an upper endoscopy, usually viewed as an ulcer or as an infectious disease. Necrosis can be found mostly between the three distals of the esophagus, but stops abruptly at the gastroesophageal junction. The basic and most common symptoms reported are blood in stool and blood in vomiting. Upper gastrointestinal bleeding then is reported, and is very commonly represented in elderly patients. Black or bloody stools and hematemesis account for over three quarters of the case presentations. Abdominal pain, nausea, vomiting, and unstable vital signs are common. A cardiovascular event (such as a heart attack) was reported in ten percent of the total known cases.

Viliuisk Encephalomyelitis (VE) is a fatal progressive neurological disorder found only in the Sakha (Iakut/Yakut) population of central Siberia. About 15 new cases are reported each year. VE is a very rare disease and little research has been conducted. The causative agents, origin of the disease, and involved candidate genes are currently unknown, but much research has been done in pursuit of the answers.

Those inflicted with the disease survive for a period of only a few months to several years. VE follows three main courses of infection: an acute form, a sub-acute form subsiding into a progressive form, and a chronic form. Initially, the infected patients experience symptoms such as: severe headaches, delirium, lethargy, meningism, bradykinesia, and incoordination. A small percentage of patients die during the acute phase as result of a severe coma. In all cases the disease is fatal.

Among the signs and symptoms of acute proliferative glomerulonephritis are the following:

- Hematuria:

- Oliguria

- Edema

- Hypertension

- Fever, headache, malaise, anorexia, nausea.

Acute esophageal necrosis can only be diagnosed by an upper gastrointestinal endoscopy.

Patients with acute GPP experience the eruption of multiple isolated sterile pustules generalized over the body, recurrent fevers, fatigue, and laboratory abnormalities (elevated ESR, elevated CRP, combined with leukocytosis).

Acute interstitial pneumonitis (also known as acute interstitial pneumonia or Hamman–Rich syndrome) is a rare, severe lung disease that usually affects otherwise healthy individuals. There is no known cause or cure.

Acute interstitial pneumonitis is often categorized as both an interstitial lung disease and a form of acute respiratory distress syndrome (ARDS) but it is distinguished from the "chronic" forms of interstitial pneumonia such as idiopathic pulmonary fibrosis.

Acute chest syndrome is often precipitated by a lung infection, and the resulting inflammation and loss of oxygen saturation leads to further sickling of red cells, thus exacerbating pulmonary and systemic hypoxemia, sickling, and vaso-occlusion.

The crisis is a common complication in sickle-cell patients and can be associated with one or more symptoms including fever, cough, excruciating pain, sputum production, shortness of breath, or low oxygen levels.

The following is a list of common signs and symptoms found with neonatal meningitis.

- Fever

- poor appetite

- anterior fontanelle bulging

- seizure

- jitteriness

- dyspnea

- irritability

- anorexia

- vomiting

- diarrhea

- abdominal distention (increase in abdominal size)

- neck rigidity

- cyanosis

- jaundice

- and sunset eyes (downward gaze of the eyes)

- abnormal body temperature (hypo-or hyperthermia)

- change of activity (lethargy or irritability)

Unfortunately these symptoms are unspecific and may point to many different conditions.

Kogoj's spongiform pustules can be observed via histopathology to confirm acute GPP.

The most common symptoms of acute interstitial pneumonitis are highly productive cough with expectoration of thick mucus, fever, and difficulties breathing. These often occur over a period of one to two weeks before medical attention is sought. The presence of fluid means the person experiences a feeling similar to 'drowning'. Difficulties breathing can quickly progress to an inability to breathe without support (respiratory failure).

Acute interstitial pneumonitis typically progresses rapidly, with hospitalization and mechanical ventilation often required only days to weeks after initial symptoms of cough, fever, and difficulties breathing develop.

In the acute stage of the disease, a catarrhal conjunctivitis is present, with signs of ocular pain, usually blepharospasm, increased lacrimation, and photophobia. Miosis is also usually present. After a few days, this will progress to a keratitis and iridocyclitis. Other ocular problems may also occur, including conjunctival and corneal oedema, and aqueous flare.

After an acute flare-up, no clinical signs of disease may be seen for a prolonged period, which can vary from a few hours to a few years. With frequent acute incidents, though, additional clinical signs may be seen, including anterior and posterior synechiae, poor pupillary responses, cataracts, and a cloudy appearance to the vitreous humour.