Characteristics of a viral infection can include pain, swelling, redness, impaired function, fever, drowsiness, confusion and convulsions.

The most common diseases caused by chronic viral infections are subacute-sclerosing panencephalitis, progressive multifocal leukoencephalopathy, retrovirus disease and spongiform encephalopathies.

Signs and symptoms may vary and some individuals may not experience any symptoms at all. The most common reported symptom of polioencephalitis is fatigue. Fatigue is associated with difficulty in attention, cognition, and maintaining wakefulness Some individuals experience psychiatric symptoms that include anxious mood, pain, insomnia, and depressed mood. Confusion and disorientation of time and space have also been reported. Motor symptoms vary more from patient to patient, but can include incoordination and tremors, nystagmus, loss of conjugate eye movements, rigidity and hemiparesis.

Onset of symptoms begins one to nine days following exposure (with an average of five). Initial symptoms include changes in taste and smell, headache, fever, nausea, vomiting, back pain, and a stiff neck. Secondary symptoms are also meningitis-like including confusion, hallucinations, lack of attention, ataxia, cramp and seizures. After the start of symptoms, the disease progresses rapidly over three to seven days, with death usually occurring anywhere from seven to fourteen days later, although it can take longer. In 2013, a man in Taiwan died twenty-five days after being infected by "Naegleria fowleri".

It affects healthy children or young adults who have recently been exposed to bodies of fresh water. Some people have presented with a clinical triad of edematous brain lesions, immune suppression, and fever.

The symptoms of cerebritis may range from mild to severe.

The severity of the symptoms varies based on the degree of swelling and on how elevated is the intracranial pressure. Mild symptoms include headaches, depression, anxiety and in some cases, memory loss. In some cases inflammation of brain can be seen if the brain or the nervous system is attacked as a result of problems with the immune system. The serious problems caused because of inflammation include headaches, seizures, vision problems, dizziness, behavior changes and even stroke.

Severe lupus cerebritis symptoms include psychosis, dementia, peripheral neuropathy, cerebellar ataxia (failure of muscular coordination, usually on one side of the body), and chorea (jerky, involuntary movements). Stroke incidence is 3-20% in systemic lupus patients, and is highest in the first five years of the disease. Peripheral neuropathy (carpal tunnel syndrome, for example) occurs in more than 20% of systemic lupus patients and cranial nerve palsies occur in 10-15%.

GAE may present in numerous ways. There is no solid definition, as only a handful of patients have presented thus far with GAE. GAE can present with: focal paralysis, seizures, brainstem symptoms, and other neurological problems, some of which may mimic glioma (especially brainstem glioma), or other brain diseases, which may hamper timely diagnosis. These symptoms are caused by inflammatory necrosis of brain tissue brought on by amoebic infiltrates.

Myelitis lesions usually occur in a narrow region but can be spread and affect many areas.

- Poliomyelitis: disease caused by viral infection in the gray matter with symptoms of muscle paralysis or weakness

- Leukomyelitis: lesions in the white matter

- Transverse Myelitis: caused by axonal demyelination encompassing both sides of the spinal cord

- Meningococcal Myelitis (or meningomyelitis): lesions occurring in the region of meninges and the spinal cord

Depending on the cause of the disease, such clinical conditions manifest different speed in progression of symptoms in a matter of hours to days. Most myelitis manifests fast progression in muscle weakness or paralysis starting with the legs and then arms with varying degrees of severity. Sometimes the dysfunction of arms or legs cause instability of posture and difficulty in walking or any movement. Also symptoms generally include paresthesia which is a sensation of tickling, tingling, burning, pricking, or numbness of a person's skin with no apparent long-term physical effect. Adult patients often report pain in the back, extremities, or abdomen. Patients also present increased urinary urgency, bowel or bladder dysfunctions such as bladder incontinence, difficulty or inability to void, and incomplete evacuation of bowel or constipation. Others also report fever, respiratory problems and intractable vomiting.

Various systems are affected.

- CNS abnormalities – microcephaly, mental retardation, spasticity, epilepsy, periventricular calcification

- Eye – choroidoretinitis and optic atrophy

- Ear – sensorineural deafness

- Liver – hepatosplenomegaly and jaundice due to hepatitis

- Lung – pneumonitis (interstitial pneumonitis)

- Heart – myocarditis

- Thrombocytopenic purpura, haemolytic anaemia

- Late sequelae in individuals asymptomatic at birth – hearing defects and reduced intelligence

Differential diagnosis may include:

- Opsoclonus-myoclonus-ataxia syndrome

- Miller-Fisher syndrome

- Meningoencephalitis

- Cerebral abscess

- Tumor

- Hydrocephalus

- Inner-ear Disease

- Acute Vestibulitis

- Acute Labyrinthitis

Cerebritis is an infection of the brain that normally leads to the formation of an abscess within the brain itself. It is the inflammation of the cerebrum, a structure within the brain, which performs a number of important functions, including most of the things which people associate with being human, such as memory and speech. It is also defined as a purulent nonencapsulated parenchymal infection of brain which is characterized by nonspecific features on CT (ill-defined low density area with peripheral enhancement) and cannot reliably be distinguished from neoplasms.

Cerebritis usually occurs as a result of an underlying condition, which causes the inflammation of the brain tissue. It is commonly found in patients with lupus. Lupus cerebritis may occur in adults and children. The duration of the central nervous system involvement may vary from a few minutes, as in classic migraine or a transient ischemic attack, to years, as in dementia. Resulting neurological deficits may be transient or permanent, occasionally resulting in death.

Naegleriasis (also known as primary amoebic meningoencephalitis) is an infection of the brain by the free-living unicellular "Naegleria fowleri".

"N. fowleri" is typically found in warm bodies of fresh water, such as ponds, lakes, rivers, and hot springs. It is also found in soil, poorly maintained municipal water supplies, water heaters, near warm-water discharges of industrial plants, and in poorly chlorinated or unchlorinated swimming pools, in an amoeboid or temporary flagellate stage. There is no evidence of it living in salt water. As the disease is rare, it is often not considered. Symptoms are similar to those of meningitis.

Although infection occurs rarely, it nearly always results in death, with a case fatality rate greater than 95%.

Most symptoms of people with post-viral cerebellar ataxia deal to a large extent with the movement of the body. Some common symptoms that are seen are clumsy body movements and eye movements, difficulty walking, nausea, vomiting, and headaches.

Granulomatous amoebic encephalitis (GAE) is a central nervous system disease caused by certain species of free-living amoebae, especially species of "Acanthamoeba" and "Balamuthia mandrillaris".

The term is most commonly used with "Acanthamoeba". In more modern references, the term "balamuthia amoebic encephalitis" (BAE) is commonly used when "Balamuthia mandrillaris" is the cause.

Immune reconstitution inflammatory syndrome (IRIS) (also known as immune recovery syndrome) is a condition seen in some cases of AIDS or immunosuppression, in which the immune system begins to recover, but then responds to a previously acquired opportunistic infection with an overwhelming inflammatory response that paradoxically makes the symptoms of infection worse.

Polioencephalitis is a viral infection of the brain, causing inflammation within the grey matter of the brain stem. The virus has an affinity for neuronal cell bodies and has been found to affect mostly the midbrain, pons, medulla and cerebellum of most infected patients. The infection can reach up through the thalamus and hypothalamus and possibly reach the cerebral hemispheres. The infection is caused by the poliomyelitis virus which is a single-stranded RNA virus surrounded by a non-enveloped capsid. Humans are the only known natural hosts of this virus. The disease has been eliminated from the U.S. since the mid-twentieth century, but is still found in certain areas of the world such as Africa.

Cytomegalic inclusion body disease (CIBD) is a series of signs and symptoms caused by cytomegalovirus infection, toxoplasmosis or other rare infections such as herpes or rubella viruses. It can produce massive calcification of the central nervous system, and often the kidneys.

Cytomegalic inclusion body disease is the most common cause of congenital abnormalities in the United States. It can also cause pneumonia and other diseases in immunocompromised patients, such as those with HIV/AIDS or recipients of organ transplants.

Neuroborreliosis is often preceded by the typical symptoms of Lyme disease, which include erythema migrans and flu-like symptoms such as fever and muscle aches. Neurologic symptoms of neuroborreliosis include the meningoradiculitis (which is more common in European patients), cranial nerve abnormalities, and altered mental status. Sensory findings may also be present. Rarely, a progressive form of encephalomyelitis may occur. In children, symptoms of neuroborreliosis include headache, sleep disturbance, and symptoms associated with increased intracranial pressure, such as papilledema, can occur. Less common childhood symptoms can include meningitis, myelitis, ataxia, and chorea. Ocular Lyme disease has also been reported, as has neuroborreliosis affecting the spinal cord, but neither of these findings are common.

The types of neurosyphilis include asymptomatic, acute syphilitic meningitis, meningovascular syphilis, parenchymatous syphilis (which includes general paresis and tabes dorsalis), and optic atrophy.

Meningeal syphilis (as known as syphilitic aseptic meningitis or meningeal neurosyphilis) is a chronic form of syphilis infection that affects the central nervous system. Treponema pallidum, which is a spirochate bacterium, is the main cause of syphilis, which spreads drastically throughout the body and can infect all the systems of the body if not treated appropriately. The bacterium is the main cause of the onset of meningeal syphilis and other treponemal diseases, and it consists of a cytoplasmic and outer membrane that can cause a diverse array of diseases in the central nervous system and brain.

Early symptomatic neurosyphillis (or acute syphilitic meningitis or neurorecurrence) is the onset of meningeal syphilis. The symptoms arise as a result of inflamed meninges, which eventually lead up to signs of meningitis.

"Treponema pallidum" invades the nervous system within three to eighteen months after the primary infection. The initial series of events is asymptomatic meningitis, which can remain in the human body system and produce more damage within the body. Every form of neurosyphilis has meningitis as a component; however, every case differs in severity. The individual is infected with syphilis through a gram negative bacteria that only humans can obtain. Syphilis has four stages of infection, which are primary, secondary, latent, and tertiary. If syphilis is not treated, the disease can affect various other systems in the body, including the brain, heart, and vessels. The infection of the heart and vessels leads to meningovascular syphilis, which is usually presented during the secondary stage of syphilis. If syphilis is prolonged, it can affect the nervous system, which is known as neurosyphilis. Meningeal syphilis is a component of neurosyphilis, which usually occurs in the tertiary stage of syphilis.

Neuroborreliosis, also known as Lyme neuroborreliosis (LNB), is a disorder of the central nervous system. A neurological manifestation of Lyme disease, neuroborreliosis is caused by a systemic infection of spirochetes of the genus "Borrelia." Symptoms of the disease include erythema migrans and flu-like symptoms. The microbiological progression of the disease is similar to that of neurosyphilis, another spirochetal infection.

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.

CNS demyelinating autoimmune diseases are autoimmune diseases which primarily affect the central nervous system.

Examples include:

- Diffuse cerebral sclerosis of Schilder

- Acute disseminated encephalomyelitis

- Acute hemorrhagic leukoencephalitis

- Multiple sclerosis (though the cause is unknown, it is sure that immune system is involved)

- Transverse myelitis

- Neuromyelitis optica

Most individuals with HSE show a decrease in their level of consciousness and an altered mental state presenting as confusion, and changes in personality. Increased numbers of white blood cells can be found in patient's cerebrospinal fluid, without the presence of pathogenic bacteria and fungi. Patients typically have a fever and may have seizures. The electrical activity of the brain changes as the disease progresses, first showing abnormalities in one temporal lobe of the brain, which spread to the other temporal lobe 7–10 days later. Imaging by CT or MRI shows characteristic changes in the temporal lobes (see Figure). Definite diagnosis requires testing of the cerebrospinal fluid (CSF) by a lumbar puncture (spinal tap) for presence of the virus. The testing takes several days to perform, and patients with suspected Herpes encephalitis should be treated with acyclovir immediately while waiting for test results.

Viremia (UK: viraemia) is a medical condition where viruses enter the bloodstream and hence have access to the rest of the body. It is similar to "bacteremia", a condition where bacteria enter the bloodstream. The name comes from combining the word virus with the Greek word for blood ("haima"). It usually lasts for 4 to 5 days in the primary condition.