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.  
Signs and symptoms
Adult patients with encephalitis present with acute onset of fever, headache, confusion, and sometimes seizures. Younger children or infants may present irritability, poor appetite and fever.  Neurological examinations usually reveal a drowsy or confused patient. Stiff neck, due to the irritation of the meninges covering the brain, indicates that the patient has either meningitis or meningoencephalitis. 
Viral encephalitis can occur either as a direct effect of an acute infection, or as one of the sequelae of a latent infection. The majority of viral cases of encephalitis have an unknown cause, however the most common identifiable cause of viral encephalitis is from herpes simplex infection.  Other causes of acute viral encephalitis are rabies virus, poliovirus, and measles virus. 
Additional possible viral causes are arbovirus (St. Louis encephalitis, West Nile encephalitis virus), bunyavirus (La Crosse strain), arenavirus (lymphocytic choriomeningitis virus) and reovirus (Colorado tick virus).  The Powassan virus is a rare cause of encephalitis.
Bacterial and other
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. 
Limbic encephalitis refers to inflammatory disease confined to the limbic system of the brain. The clinical presentation often includes disorientation, disinhibition, memory loss, seizures, and behavioral anomalies. MRI imaging reveals T2 hyperintensity in the structures of the medial temporal lobes, and in some cases, other limbic structures. Some cases of limbic encephalitis are of autoimmune origin. 
Autoimmune encephalitis signs can include catatonia, psychosis, abnormal movements, and autonomic dysregulation. Antibody-mediated anti-N-methyl-D-aspartate-receptor encephalitis and Rasmussen encephalitis are examples of autoimmune encephalitis. 
Encephalitis lethargica is identified by high fever, headache, delayed physical response, and lethargy. Individuals can exhibit upper body weakness, muscular pains, and tremors, though the cause of encephalitis lethargica is not currently known. From 1917 to 1928, an epidemic of encephalitis lethargica occurred worldwide. 
People should only be diagnosed with encephalitis if they have a decreased or altered level of consciousness, lethargy, or personality change for at least twenty-four hours without any other explainable cause.  Diagnosing encephalitis is done via a variety of tests: 
- Brain scan, done by MRI, can determine inflammation and differentiate from other possible causes.
- EEG, in monitoring brain activity, encephalitis will produce abnormal signal.
- Lumbar puncture (spinal tap), this helps determine via a test using the cerebral-spinal fluid, obtained from the lumbar region.
- Blood test
- Urine analysis
- Polymerase chain reaction (PCR) testing of the cerebrospinal fluid, to detect the presence of viral DNA which is a sign of viral encephalitis. 
Vaccination is available against tick-borne  and Japanese encephalitis  and should be considered for at-risk individuals. Post-infectious encephalomyelitis complicating smallpox vaccination is avoidable, for all intents and purposes, as smallpox is nearly eradicated. Contraindication to Pertussis immunization should be observed in patients with encephalitis.
Treatment (which is based on supportive care) is as follows: 
Pyrimethamine -based maintenance therapy is often used to treat Toxoplasmic Encephalitis (TE), which is caused by Toxoplasma gondii and can be life-threatening for people with weak immune systems.  The use of highly active antiretroviral therapy (HAART), in conjunction with the established pyrimethamine-based maintenance therapy, decreases the chance of relapse in patients with HIV and TE from approximately 18% to 11%.  This is a significant difference as relapse may impact the severity and prognosis of disease and result in an increase in healthcare expenditure. 
Identification of poor prognostic factors include thrombocytopenia, cerebral edema, status epilepticus, and thrombocytopenia.  In contrast, a normal encephalogram at the early stages of diagnosis is associated with high rates of survival. 
The number of new cases a year of acute encephalitis in Western countries is 7.4 cases per 100,000 population per year. In tropical countries, the incidence is 6.34 per 100,000 per year.  The incidence of Encephalitis has not changed much over time, with an incidence of encephalitis in the US of 250,000 from 2005 to 2015. Approximately seven per 100,000 patients were hospitalized for encephalitis in the US during this time.  In 2015, encephalitis was estimated to have affected 4.3 million people and resulted in 150,000 deaths worldwide.   Herpes simplex encephalitis has an incidence of 2–4 per million population per year.