The number of new cases a year is unknown. According to the California Encephalitis Project, the disease has a higher incidence than its individual viral counterparts in patients younger than 30. The largest case series to date characterized 577 patients with anti-NMDA receptor encephalitis. The epidemiological data were limited, but this study provides the best approximation of disease distribution. It found that women are disproportionally affected, with 81% of cases reported in female patients. Disease onset is skewed toward children, with a median age of diagnosis of 21 years. Over a third of cases were children, while only 5% of cases were patients over the age of 45. This same review found that 394 out of 501 patients (79%) had a good outcome by 24 months. 30 patients (6%) died, and the rest were left with mild to severe deficits. The study also confirmed that patients with the condition are more likely to be of Asian or African origin.

Limbic encephalitis is associated with an autoimmune reaction. In non-paraneoplastic limbic enephalitis, this is typically due to infection (commonly herpes simplex virus) or as a systemic autoimmune disorder. Limbic encephalitis associated with cancer or tumors is called paraneoplastic limbic encephalitis.

Prognosis is poor, however, current analysis suggests that those associated with thymoma, benign or malignant, show a less favorable prognosis (CASPR2 Ab positive).

Most common cause of autoimmune encephalitis after acute demyelinating encephalitis in England. More than 500 cases have been reported in literature till 2013. In California Encephalitis Project it was found >4 times as frequently as herpes simplex virus type 1 (HSV-1), varicella-zoster virus (VZV), and West Nile virus (WNV). Among patients with first-onset schizophrenia incidence varies between 6–10%.

- Age – frequently 5–76 years, Median age of patients was 23 years

- Sex – 80% Female

Limbic encephalitis is broadly grouped into two types: paraneoplastic limbic encephalitis and non-paraneoplastic limbic encephalitis.

- Paraneoplastic limbic encephalitis (PNLE) is caused by cancer or tumor, and may be treated by removal of the tumor.

- Non-paraneoplastic limbic encephalitis (NPLE) is not associated with cancer. More common than PNLE, it is caused by am infection, autoimmune disorder, or other condition that may never be identified.

Anti-NMDA receptor encephalitis, also known as NMDA receptor antibody encephalitis, is an acute form of brain inflammation that is potentially lethal but has a high probability for recovery with treatment.

It is caused by an immune system attack, primarily targeting the NR1 subunit of the NMDA receptor (N-methyl D-aspartate receptor). The condition is associated with tumors, mostly teratomas of the ovaries. However, many cases are not associated with tumors.

The disease was officially categorized and named by Josep Dalmau and colleagues in 2007.

The causes of encephalitis lethargica (EL) are uncertain.

Veins of modern research have explored its origins in an autoimmune response, and, separately or in relation to an immune response, links to pathologies of infectious disease (viral and bacterial, e.g., in the case of influenza, where a link with encephalitis is clear). Postencephalic Parkinsonism was clearly documented to have followed an outbreak of EL following 1918 influenza pandemic; evidence for viral causation of the Parkinson's symptoms is circumstantial (epidemiologic, and finding influenza antigens in EL patients), while evidence arguing against this cause is of the negative sort (e.g., lack of viral RNA in postencephalic parkinsonian brain material).

In reviewing the relationship between influenza and EL, McCall and coworkers conclude, as of 2008, that while "the case against influenza [is] less decisive than currently perceived… there is little direct evidence supporting influenza in the etiology of EL," and that "[a]lmost 100 years after the EL epidemic, its etiology remains enigmatic." Hence, while opinions on the relationship of EL to influenza remain divided, the preponderance of literature appears skeptical.

In 2010, in a substantial Oxford University Press compendium reviewing the historic and contemporary views on EL, its editor, Joel VIlensky of the Indiana University School of Medicine, quotes Pool, writing in 1930, who states, "we must confess that etiology is still obscure, the causative agent still unknown, the pathological riddle still unsolved…", and goes on to offer the following conclusion, as of that publication date:Subsequent to publication of this compendium, an enterovirus was discovered in EL cases from the epidemic.

Diplococcus has been implicated as a cause of EL.

The clinical features and course of the condition, the associated auto-antibodies against relevant antigens, and the response to treatment, all suggest that Bickerstaff brainstem encephalitis is an autoimmune disease. However, each of these criteria fails to fit a substantial proportion of patients, and there is no single test or feature which is diagnostic of Bickerstaff brainstem encephalitis. It is therefore possible that a proportion of cases are due to other causes, such as infection or lymphoma, but remain undiagnosed. It is also possible that there is more than one autoimmune disease that can cause an illness which would currently be diagnosed as Bickerstaff's. There is certainly overlap between Guillain–Barré syndrome, Miller Fisher syndrome and Bickerstaff brainstem encephalitis, as well as other conditions associated with anti-ganglioside antibodies such as chronic ophthalmoplegia with anti-GQ1b antibody.

and the pharyngo-cervico-brachial variant of GBS.

It can be caused by a bacterial infection, such as bacterial meningitis, or may be a complication of a current infectious disease syphilis (secondary encephalitis).

Certain parasitic or protozoal infestations, such as toxoplasmosis, malaria, or primary amoebic meningoencephalitis, can also cause encephalitis in people with compromised immune systems. Lyme disease or "Bartonella henselae" may also cause encephalitis.

Other bacterial pathogens, like "Mycoplasma" and those causing rickettsial disease, cause inflammation of the meninges and consequently encephalitis. A non-infectious cause includes acute disseminated encephalitis which is demyelinated.

Encephalitis is inflammation of the brain. Severity is variable. Symptoms may include headache, fever, confusion, a stiff neck, and vomiting. Complications may include seizures, hallucinations, trouble speaking, memory problems, and problems with hearing.

Causes of encephalitis include viruses such as herpes simplex virus or rabies, bacteria, fungus, or parasites. Other causes include autoimmune diseases and certain medication. In many cases the cause remains unknown. Risk factors include a weak immune system. Diagnosis is typically based on symptoms and supported by blood tests, medical imaging, and analysis of cerebrospinal fluid.

Certain types are preventable with vaccines. Treatment may include, antiviral medication (such as acyclovir), anticonvulsants, and corticosteroids. Treatment generally takes place in hospital. Some people require artificial respiration. Once the immediate problem is under control, rehabilitation may be required. In 2015, encephalitis was estimated to have affected 4.3 million people and resulted in 150,000 deaths worldwide.

There have been several proposed diagnostic criteria for Encephalitis Lethargica. One, which has been widely accepted, includes an acute or subacute encephalitic illness where all other known causes of encephalitis have been excluded. Another diagnostic criterion, suggested more recently,says that the diagnosis of Encephalitis Lethargica "may be considered if the patient’s condition cannot be attributed to any other known neurological condition and that they show the following signs: influenza-like signs; hypersomnolence (hypersomnia), wakeability, opthalmoplegia (paralysis of the muscles that control the movement of the eye), and psychiatric changes."

In Rasmussen’s encephalitis, there is chronic inflammation of the brain, with infiltration of T lymphocytes into the brain tissue. In most cases, this affects only one cerebral hemisphere, either the left or the right. This inflammation causes permanent damage to the cells of the brain, leading to atrophy of the hemisphere; the epilepsy that this causes may itself contribute to the brain damage. The epilepsy might derive from a disturbed GABA release, the main inhibitory neurotransmitter of the mammalian brain.

The cause of the inflammation is not known: infection by a virus has been suggested, but the evidence for this is inconclusive. In the 1990s it was suggested that auto-antibodies against the glutamate receptor GluR3 were important in causing the disease, but this is no longer thought to be the case. However, more recent studies report the presence of autoantibodies against the NMDA-type glutamate receptor subunit GluRepsilon2 (anti-NR2A antibodies) in a subset of patients with Rasmussen's encephalitis. There has also been some evidence that patients suffering from RE express auto-antibodies against alpha 7 subunit of the nicotinic acetylcholine receptor. By sequencing T cell receptors from various compartments it could be shown that RE patients present with peripheral CD8+ T-cell expansion which in some cases have been proven for years after disease onset.

Rasmussen's encephalitis has been recorded with a neurovisceral porphyria, acute intermittent porphyria and after ADEM (acute disseminated encephalomyelitis).

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.

Meningitis is a very common in children. Newborns can develop herpes virus infections through contact with infected secretions in the birth canal. Other viral infections are acquired by breathing air contaminated with virus-containing droplets exhaled by an infected person. Arbovirus infections are acquired from bites by infected insects (called epidemic encephalitis). Viral central nervous system infections in newborns and infants usually begin with fever. The inability of infants to communicate directly makes it difficult to understand their symptoms. Newborns may have no other symptoms and may initially not otherwise appear ill. Infants older than a month or so typically become irritable and fussy and refuse to eat. Vomiting is common. Sometimes the soft spot on top of a newborn's head (fontanelle) bulges, indicating an increase in pressure on the brain. Because irritation of the meninges is worsened by movement, an infant with meningitis may cry more, rather than calm down, when picked up and rocked. Some infants develop a strange, high-pitched cry. Infants with encephalitis often have seizures or other abnormal movements. Infants with severe encephalitis may become lethargic and comatose and then die. To make the diagnosis of meningitis or the diagnosis of encephalitis, doctors do a spinal tap (lumbar puncture) to obtain cerebrospinal fluid (CSF) for laboratory analysis in children.

Viral encephalitis is a type of encephalitis caused by a virus.

It is unclear if anticonvulsants used in people with viral encephalitis would prevent seizures.

During the acute stage, treatment is aimed at reducing the inflammation. As in other inflammatory diseases, steroids may be used first of all, either as a short course of high-dose treatment, or in a lower dose for long-term treatment. Intravenous immunoglobulin is also effective both in the short term and in the long term, particularly in adults where it has been proposed as first-line treatment. Other similar treatments include plasmapheresis and tacrolimus, though there is less evidence for these. None of these treatments can prevent permanent disability from developing.

During the residual stage of the illness when there is no longer active inflammation, treatment is aimed at improving the remaining symptoms. Standard anti-epileptic drugs are usually ineffective in controlling seizures, and it may be necessary to surgically remove or disconnect the affected cerebral hemisphere, in an operation called hemispherectomy. This usually results in further weakness, hemianopsia and cognitive problems, but the other side of the brain may be able to take over some of the function, particularly in young children. The operation may not be advisable if the left hemisphere is affected, since this hemisphere contains most of the parts of the brain that control language. However, hemispherectomy is often very effective in reducing seizures.

Herpesviral Encephalitis can be treated with high-dose intravenous acyclovir. Without treatment, HSE results in rapid death in approximately 70% of cases; survivors suffer severe neurological damage. When treated, HSE is still fatal in one-third of cases, and causes serious long-term neurological damage in over half of survivors. Twenty percent of treated patients recover with minor damage. Only a small population of survivors (2.5%) regain completely normal brain function. Indeed, many amnesic cases in the scientific literature have etiologies involving HSE. Earlier treatment (within 48 hours of symptom onset) improves the chances of a good recovery. Rarely, treated individuals can have relapse of infection weeks to months later. There is evidence that aberrant inflammation triggered by herpes simplex can result in granulomatous inflammation in the brain, which responds to steroids. While the herpes virus can be spread, encephalitis itself is not infectious. Other viruses can cause similar symptoms of encephalitis, though usually milder (Herpesvirus 6, varicella zoster virus, Epstein-Barr, cytomegalovirus, coxsackievirus, etc.).

Many viral infections of the central nervous system occur in seasonal peaks or as epidemics, whereas others, such as herpes simplex encephalitis, are sporadic. In endemic areas it is mostly a disease of children, but as the disease spreads to new regions, or nonimmune travelers visit endemic regions, nonimmune adults are also affected.

Herpesviral encephalitis is encephalitis due to herpes simplex virus.

Herpes simplex encephalitis (HSE) is a viral infection of the human central nervous system. It is estimated to affect at least 1 in 500,000 individuals per year and some studies suggest an incidence rate of 5.9 cases per 100,000 live births. The majority of cases of herpes encephalitis are caused by herpes simplex virus-1 (HSV-1), the same virus that causes cold sores. 57% of American adults are infected with HSV-1, which is spread through droplets, casual contact, and sometimes sexual contact, though most infected people never have cold sores. About 10% of cases of herpes encephalitis are due to HSV-2, which is typically spread through sexual contact. About 1 in 3 cases of HSE result from primary HSV-1 infection, predominantly occurring in individuals under the age of 18; 2 in 3 cases occur in seropositive persons, few of whom have history of recurrent orofacial herpes. Approximately 50% of individuals who develop HSE are over 50 years of age.

In the United States each year approximately 1,000,000 individuals develop herpes zoster. Of those individuals approximately 10-18% develop postherpetic neuralgia.

Less than 10 percent of people younger than 60 develop postherpetic neuralgia after a bout of herpes zoster, while about 40 percent of people older than 60 do.

It is transmitted by the bite of several species of infected ticks, including "Ixodes scapularis", "I. ricinus" and "I. persulcatus", or (rarely) through the non-pasteurized milk of infected cows.

Antibodies against voltage-gated potassium channels (VGKC), which are detectable in about 40% of patients with acquired neuromytonia, have been implicated in Morvan’s pathophysiology. Raised serum levels of antibodies to VGKCs have been reported in three patients with Morvan’s Syndrome. Binding of serum from a patient with Morvan’s Syndrome to the hippocampus in a similar pattern of antibodies to known VGKC suggest that these antibodies can also cause CNS dysfunction. Additional antibodies against neuromuscular junction channels and receptors have also been described. Experimental evidence exists that these anti-VGKC antibodies cause nerve hyperexcitability by suppression of voltage gated K+ outward currents, whereas other, yet undefined humoral factors have been implicated in anti-VGKC antibody negative neuromyotonia. It is believed that antibodies to the Shaker-type K+ channels (the Kv1 family) are the type of potassium channel most strongly associated with acquired neuromyotonia and Morvan’s Syndrome.

Whether VGKC antibodies play a pathogenic role in the encephalopathy as they do in the peripheral nervous system is as yet unclear. It has been suggested that the VGKC antibodies may cross the blood–brain barrier and act centrally, binding predominantly to thalamic and striatal neurons causing encephalopathic and autonomic features.

Arbovirus encephalitis refers to encephalitis that is caused by arbovirus infection.

There are many types of arboviral encephalitides found in the United States.

Examples include:

- California encephalitis

- Japanese encephalitis

- St. Louis encephalitis

- Tick-borne encephalitis

- West Nile fever

- Murray Valley encephalitis

In one case, a patient was diagnosed with both Morvan's syndrome and pulmonary hyalinizing granulomas (PHG). PHG are rare fibrosing lesions of the lung, which have central whorled deposits of lamellar collagen. How these two diseases relate to one another is still unclear.

Thymoma, prostate adenoma, and in situ carcinoma of the sigmoid colon have also been found in patients with Morvan’s Syndrome.

The disease is most common in Central and Eastern Europe, and Northern Asia. About ten to twelve thousand cases are documented a year but the rates vary widely from one region to another. Most of the variation is the result of variation in host population, particularly that of deer. In Austria, an extensive free vaccination program since the 1960s has reduced incidence by roughly 85%. In Sweden, most cases of TBE occur in a band running from Stockholm to the west, especially around lakes and the nearby region of the Baltic sea. This reflects the greater population involved in outdoor activities in these areas. Although in some regions of Russia and Slovenia the prevalence of cases can be as high as 70 cases per 100,000 people per year, in most regions it is far lower, and overall, for Europe the estimated risk is roughly 1 case per 10,000 human-months of woodland activity. Travelers to endemic regions do not experience many cases, with only 5 cases reported among U.S. travelers returning from Eurasia between 2000 and 2011, a rate so low that the U.S. Centers for Disease Control and Prevention recommend vaccination only for those who will be extensively exposed in high risk areas.