Causes

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.

Infections

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:

• herpes simplex viruses, which cause Herpes simplex virus and genital herpes (this is the most common cause of encephalitis)
• the varicella zoster virus, which causes chickenpox and shingles
• measles , mumps and rubella viruses
• viruses spread by animals, such as tick-borne encephalitis , Japanese encephalitis , rabies (and possibly Zika virus )

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.

Problems with the immune system

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:

• a previous infection in another part of the body (which usually occurs a few weeks earlier), such as one oftheinfections mentioned above
• anon-cancerous or cancerous growth (tumour) somewhere in the body
• a vaccination (this is very rare and the benefits of being vaccinated far outweigh the risk of encephalitis)
• an unknown cause

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".