It's not always clear what causes encephalitis. When a cause is found, it's usually an infection or a problem with the immune system (the body's natural defences).
Some of the main causes are outlined below.
Encephalitis can occur if an infection spreads to the brain.
Many of the infections associated with the condition are quite common and are usually mild. Encephalitis only occurs in rare cases.
The condition is most often due toa virus, such as:
Encephalitis caused by a virus is known as "viral encephalitis". In rare cases,encephalitisis caused by bacteria, fungi or parasites.
You can catch these infections from someone else, but encephalitis itself isn't spread from person to person.
The immune system protectsthe body from illness and infection. When germs enter the body, the immune system attacks them to stop them causing a serious infection.
But very rarely something goes wrong with the immune system and it mistakenly attacks the brain,causing encephalitis.
This can be triggered by:
Encephalitis due to a reaction to a previous infection is known as "post-infectious encephalitis". If it's caused by a tumour or the cause is unknown, it's called "autoimmune encephalitis".
Find out about encephalitis, a rare but serious condition that causes inflammation of the brain.
Find out about the main signs and symptoms of encephalitis, and when to get medical advice.
Find out about the main causes of encephalitis, including viral infections and problems with the immune system.
Find out about the tests that may need to be carried out to diagnose encephalitis, including brain scans and a lumbar puncture.
Find out how and where encephalitis is treated.
Find out about the further problems that can occur after encephalitis, and what help and support is available.