ADEM has an abrupt onset and a monophasic course. Symptoms usually begin 1–3 weeks after infection. Major symptoms include fever, headache, nausea and vomiting, confusion, vision impairment, drowsiness, seizures and coma. Although initially the symptoms are usually mild, they worsen rapidly over the course of hours to days, with the average time to maximum severity being about four and a half days. Additional symptoms include hemiparesis, paraparesis, and cranial nerve palsies.

Acute disseminated encephalomyelitis (ADEM), or acute demyelinating encephalomyelitis, is a rare autoimmune disease marked by a sudden, widespread attack of inflammation in the brain and spinal cord. As well as causing the brain and spinal cord to become inflamed, ADEM also attacks the nerves of the central nervous system and damages their myelin insulation, which, as a result, destroys the white matter. It is often triggered after the patient has received a viral infection or, perhaps exceedingly rarely specific non-routine vaccinations.

ADEM's symptoms resemble the symptoms of multiple sclerosis (MS), so the disease itself is sorted into the classification of the multiple sclerosis borderline diseases. However, ADEM has several features that distinguish it from MS. Unlike MS, ADEM occurs usually in children and is marked with rapid fever, although adolescents and adults can get the disease too. ADEM consists of a single flare-up whereas MS is marked with several flare-ups (or relapses), over a long period of time. Relapses following ADEM are reported in up to a quarter of patients, but the majority of these 'multiphasic' presentations following ADEM likely represent MS. ADEM is also distinguished by a loss of consciousness, coma and death, which is very rare in MS, except in severe cases.

The incidence rate is about 8 per 1,000,000 people per year. Although it occurs in all ages, most reported cases are in children and adolescents, with the average age around 5 to 8 years old. The disease affects males and females almost equally. The mortality rate may be as high as 5%; however, full recovery is seen in 50 to 75% of cases with increase in survival rates up to 70 to 90% with figures including minor residual disability as well. The average time to recover from ADEM flare-ups is one to six months.

ADEM produces multiple inflammatory lesions in the brain and spinal cord, particularly in the white matter. Usually these are found in the subcortical and central white matter and cortical gray-white junction of both cerebral hemispheres, cerebellum, brainstem, and spinal cord, but periventricular white matter and gray matter of the cortex, thalami and basal ganglia may also be involved.

When the patient suffers more than one demyelinating episode of ADEM, the disease is then called recurrent disseminated encephalomyelitis or multiphasic disseminated encephalomyelitis (MDEM). Also, a fulminant course in adults has been described.

The conjunctiva provides lining for the inside of the eyelid as well as a coating for the sclera, the white portion of the eyeball. It typically serves to provide lubrication for the eye through the production of mucus and tears. When infected with AHC, patients will experience painful, red eyes, swelling of the conjunctival tissue, and frequent mucus discharge from the eyes accompanied by excessive tearing and subconjunctival hemorrhaging. This hemorrhaging is caused by the rupture of blood vessels beneath the conjunctiva giving the eyes a bright red appearance. Some patients also experience blurry vision and fever in addition to the common symptoms and one in every ten thousand cases exhibits paralysis similar to that of polio.

Acute hemorrhagic conjunctivitis (AHC) (also spelled acute haemorrhagic conjunctivitis) is a derivative of the highly contagious conjunctivitis virus, otherwise known as pink eye. Symptoms include excessively red, swollen eyes as well as subconjuntival hemorrhaging. Currently, there is no known treatment and patients are required to merely endure the symptoms while the virus runs its five- to seven-day course. While it was first identified in Ghana, the virus has now been seen in China, India, Egypt, Cuba, Singapore, Taiwan, Japan, Pakistan, Thailand, and the United States.

Nephropathia epidemica ("NE" or epidemic nephropathy) is a type of viral haemorrhagic fever with renal syndrome (HFRS) caused by the Puumala virus. The incubation period is three weeks. Nephropathia epidemica has a sudden onset with fever, abdominal pain, headache, back pain and gastrointestinal symptoms. More severe symptoms include internal hemorrhaging. Although fatal in a small percentage of cases, nephropathia epidemica is generally milder than the HFRS that is caused by hantaviruses in other parts of the world. The bank vole is the reservoir for the virus, which humans contract through inhalation of aerosolised vole droppings.

The majority of infected individuals are asymptomatic or develop only mild symptoms, and the disease is not known to spread from human to human.

This infection is known as "sorkfeber" in Swedish and "myyräkuume" in Finnish (vole fever). In Norway, it is called "musepest" (mouse plague).

The signs shown depend on the horse's age, the strain of the infecting virus, the condition of the horse and the route by which it was infected. Most horses with EVA infection don't show any signs; if a horse does show symptoms, these can vary greatly in severity. Following infection, the first sign is fever, peaking at , followed by various signs such as depression, nasal discharge, "pink eye" (conjunctivitis), swelling over the eye (supraorbital edema), urticaria, and swelling of the limbs and under the belly (the ventral abdomen) which may extend to the udder in mares or the scrotum of male horses. More unusual signs include abortion in pregnant mares, and, most likely in foals, severe respiratory distress and death.

The symptoms of the disease include a high fever with frontal headaches, followed by haemorrhagic symptoms, such as bleeding from the nasal cavity, throat, and gums, as well as gastrointestinal bleeding. Other symptoms include vomiting, muscle stiffness, tremors, absent reflexes, and mental disturbances.

An affected person may recover in two weeks time, but the convalescent period is typically very long, lasting for several months. There will be muscle aches and weakness during this period and the affected person is unable to engage in physical activities.

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

The disease has a fatality rate of 3-10%, and it affects 400-500 people annually.

Equine viral arteritis (EVA) is a disease of horses caused by equine arteritis virus, an RNA virus of the genus "Arterivirus". The virus which causes EVA was first isolated in 1953, but the disease has afflicted equine animals worldwide for centuries. It has been more common in some breeds of horses in the United States, but there is no breed "immunity". In the UK, it is a notifiable disease. There is no known human hazard.

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

- Hematuria:

- Oliguria

- Edema

- Hypertension

- Fever, headache, malaise, anorexia, nausea.

Acute Cerebellar ataxia is a diagnosis of exclusion. Urgent CT scan is necessary to rule out cerebellar tumor or hemorrhage as cause of the ataxia; however in acute cerebellar ataxia, the CT will be normal. CSF studies are normal earlier in the course of disease. Later on CSF shows moderate elevation of proteins.

Anterior uveitis develops in 40–50% of cases with HZO within 2 weeks of onset of the skin rashes. Typical HZO keratitis at least mild iritis, especially if Hutchinson's sign is positive for the presence of vescicles upon the tip of the nose.

Features:

This non-granulomatous iridocyclitis is associated with:

- Small keratic precipitates

- Mild aqueous flare

- Occasionally haemorrhagic hypopion.

HZO uveitis is associated with complications such as iris atrophy and secondary glaucoma are not uncommon. Complicated cataract may develop in the late stages of the disease.

Any potential ocular involvement should be assessed by an ophthalmologist as complications such as episcleritis and uveitis may occur.

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.

Acute proliferative glomerulonephritis is a disorder of the glomeruli (glomerulonephritis), or small blood vessels in the kidneys. It is a common complication of bacterial infections, typically skin infection by "Streptococcus" bacteria types 12, 4 and 1 (impetigo) but also after streptococcal pharyngitis, for which it is also known as "postinfectious" or poststreptococcal glomerulonephritis. It can be a risk factor for future albuminuria. In adults, the signs and symptoms of infection may still be present at the time when the kidney problems develop, and the terms infection-related glomerulonephritis or bacterial infection-related glomerulonephritis are also used. Acute glomerulonephritis resulted in 19,000 deaths in 2013 down from 24,000 deaths in 1990 worldwide.

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).

The clinical hallmark is haemorrhagic bullae on the mucosa of the oronasopharynx. Haemorrhage from ruptured bullae, epistaxis or gastrointestinal bleeding is severe and may cause shock and death.

Onyalai is an acute disease that results in the development of hemorrhagic lesions on oral, nasal, and subconjunctival mucous membranes and skin, including on the soles of the feet. The patient initially is not in distress, which may result in a delay of diagnosis. As the disease progresses, hematuria, melena and menorrhagia may develop. Bleeding usually persists for approximately eight days, and may recur. Approximately 80 percent of cases will develop chronic thrombocytopenia. Periodic episodes of acute hemorrhage are possible and may be severe, leading to shock and death.

Historically, six "classical" infectious childhood exanthems have been recognized, four of which are viral. Numbers were provided in 1905.

The four viral exanthema have much in common, and are often studied together as a class. They are:

Scarlet fever, or "second disease", is associated with the bacterium "Streptococcus pyogenes". Fourth disease, a condition whose existence is not widely accepted today, was described in 1900 and is postulated to be related to the bacterium "Staphylococcus aureus".

Many other common viruses apart from the ones mentioned above can also produce an exanthem as part of their presentation, though they are not considered part of the classic numbered list:

- Varicella zoster virus (chickenpox or shingles)

- Mumps

- rhinovirus (the common cold)

- unilateral laterothoracic exanthem of childhood

- Some types of viral haemorrhagic fever are also known to produce a systemic rash of this kind during the progression of the disease.

- Tick-borne diseases like Rocky Mountain spotted fever produce a rash that may become extensive enough so as to be classified as exanthemous in as many as 90% of children with the disease.

An exanthem or exanthema (from Greek ἐξάνθημα "exánthēma", "a breaking out") is a widespread rash usually occurring in children. An exanthem can be caused by toxins, drugs, or microorganisms, or can result from autoimmune disease.

It can be contrasted with an enanthem.

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

In the acute form of the disease caused by highly virulent strains, pigs may develop a high fever, but show no other noticeable symptoms for the first few days. They then gradually lose their appetites and become depressed. In white-skinned pigs, the extremities turn blueish-purple and hemorrhages become apparent on the ears and abdomen. Groups of infected pigs lie huddled together shivering, breathing abnormally, and sometimes coughing. If forced to stand, they appear unsteady on their legs. Within a few days of infection, they enter a comatose state and then die. In pregnant sows, spontaneous abortions occur. In milder infections, affected pigs lose weight, becoming thin, and develop signs of pneumonia, skin ulcers, and swollen joints.

Malarial nephropathies are reported in endemic areas, such as Southeast Asia, India, and Sub-Saharan Africa. The pathogenesis of acute renal failure in severe malaria is unspecific and multifactorial—it affects fewer than 4.8 percent of cases, but reports a high risk of mortality (15 to 45 percent). Histologic evidence shows a large combination of pathogenic mechanisms at play—acute tubular necrosis, interstitial nephritis and glomerulonephritis. Risk factors for malarial acute renal failure include delayed diagnosis, high parasitemia, and clinical presentation of oliguria, low blood pressure, severe anemia, and jaundice. In addition, patients already suffering from diarrhea, hepatitis, or respiratory distress have a worse prognosis.

Onyalai is an acquired form of immune thrombocytopenia which differs clinically, epidemiologically and immunologically from idiopathic thrombocytopenic purpura.