[5] The severity can be variable with symptoms including reduction or alteration in consciousness, aphasia, headache, fever, confusion, a stiff neck, and vomiting.

[2] Diagnosis is typically based on symptoms and supported by blood tests, medical imaging, and analysis of cerebrospinal fluid.

The clinical presentation often includes disorientation, disinhibition, memory loss, seizures, and behavioral anomalies.

Individuals can exhibit upper body weakness, muscular pains, and tremors, though the cause of encephalitis lethargica is not currently known.

[citation needed] Certain parasitic or protozoal infestations, such as toxoplasmosis and malaria can also cause encephalitis in people with compromised immune systems.

[20] Anti-NMDA receptor encephalitis is the most common autoimmune form, and is accompanied by ovarian teratoma in 58 percent of affected women 18–45 years of age.

[21] Another autoimmune cause includes acute disseminated encephalitis, a demyelinating disease which primarily affects children.

[22] 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.

[29] An ideal drug to treat brain infection should be small, moderately lipophilic at pH of 7.4, low level of plasma protein binding, volume of distribution of litre per kg, does not have strong affinity towards binding with P-glycoprotein, or other efflux pumps on the surface of blood–brain barrier.

Some drugs such as isoniazid, pyrazinamide, linezolid, metronidazole, fluconazole, and some fluoroquinolones have good penetration to blood brain barrier.

[30] Treatment (which is based on supportive care) is as follows:[31] 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.

[32] This is a significant difference as relapse may impact the severity and prognosis of disease and result in an increase in healthcare expenditure.

[2] The word is from Ancient Greek ἐγκέφαλος, enképhalos 'brain',[37] composed of ἐν, en, 'in' and κεφαλή, kephalé, 'head', and the medical suffix -itis 'inflammation'.