Encephalitis is an inflammation of the brain. People with encephalitis usually experience mild symptoms, if any. However, encephalitis can be severe or even fatal in some circumstances.

Encephalitis is acquired by either infection of the brain and spinal cord (primary encephalitis) or as a result of an infection elsewhere in the body that spreads to the brain (secondary or postinfectious encephalitis). Primary encephalitis tends to be more dangerous, although it is much less common than secondary encephalitis. In general, encephalitis is rare in the United States.

Many viruses may cause encephalitis, including several viruses in the herpes group such as herpes simplex virus 1 (HSV-1), herpes simplex virus 2 (HSV-2) and chickenpox. Other viruses that can cause encephalitis are enteroviruses, which are usually found in human intestines and arboviruses, which are transmitted by infected arthropods (insects and insect-like animals with a hard outer skeleton and jointed limbs), usually mosquitoes.

Of the types of encephalitis caused by arboviruses, Japanese encephalitis is the most common form of the disease worldwide. While it is the cause of thousands of cases in Asia each year, it is rare in the United States. Occasionally there are notable outbreaks of diseases caused by arboviruses such as the West Nile virus or eastern equine encephalitis. In addition, bacteria, fungi and parasites may also cause encephalitis if they invade the brain.

Encephalitis frequently occurs with meningitis. In these cases, inflammation affects the meninges, the membranes that line the brain and spinal cord. Both diseases have similar routes to the body and similar symptoms.

 Encephalitis mainly affects children, elderly people, individuals with weakened immune systems (e.g., cancer or HIV/ AIDS patients) or people who reside in areas with high mosquito populations.

Pain and discomfort (e.g., headache, neck or back stiffness) are common symptoms of encephalitis. However, the type and severity of symptoms can vary greatly among individuals.

Methods for diagnosing encephalitis include spinal tap, blood tests and brain imaging. Depending on the severity of the disease in the patient, treatment options may involve rest, physical therapy, speech therapy, antibiotics and anticonvulsant (seizure) medications.