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).

Encephalitis can happen if an infection spreads to the brain.

Many of the infections associated with the condition are quite common and are usually mild. Encephalitis only happens in rare cases.

Encephalitis is most often due to a virus, such as:

• herpes simplex viruses, which cause cold sores (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, encephalitis is caused by bacteria, fungi or parasites.

You can catch these infections from someone else, but encephalitis itself is not spread from person to person.

The immune system protects the 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 happens a few weeks earlier)
• a non-cancerous or cancerous growth (tumour) somewhere in the body
• a vaccination (this is very rare and the benefits of vaccination far outweigh the risk of encephalitis)

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".