This is the question that I really cannot even begin to answer. The answer is that no one knows, it is similar to asking “why did I get hit by a bus?”.

I can, however, explain the different paths that lead to encephalitis.

There are many different ‘types’ of encephalitis, therefore there are many different causes which lead to the inflammation in the brain. There are far too many too list, therefore I am going to keep it simple.

One way to compartmentalise the different types are into primary and secondary encephalitis.

• Primary encephalitis -occurs when a virus or other infectious agent directly infects the brain. The infection may be concentrated in one area or widespread. A primary infection may be a reactivation of a virus that had been inactive (latent) after a previous illness.

Viruses are the most common agents that cause infectious encephalitis.

Within the British Isles herpes simplex virus (the cold sore virus) is the virus most frequently identified.

Worldwide other viruses are responsible, many of which are transmitted by a mosquito bite, although in some cases the infection is never determined despite testing.

Known types of  infectious encephalitis: Herpes simplex virus, West Nile, tick borne, Japanese, Mycoplasma, Enteroviral, California, St Louis, Western Equine, Eastern Equine, Murray Valley,  La Crosse, Colorado Tick Fever.

• Secondary encephalitis – is a faulty immune system reaction in response to an infection elsewhere in the body. Instead of solely attacking the cells causing an infection, the immune system also mistakenly attacks healthy cells in the brain.

Secondary encephalitis often occurs two to three weeks after the initial infection.

Rarely, secondary encephalitis occurs as a complication of a live virus vaccination.

Known types of post-infectious encephalitis: Acute Disseminated Encephalomyelitis (ADEM), Voltage-gated Potassium Channel-complex Antibody-associated limbic encephalitis, Hashimoto’s Encephalopathy (HE), Rasmussen Encephalitis (RE) and anti NMDA-r encephalitis.

More information about the causes of encephalitis can be found at the Encephalitis Society website.

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Disclaimer: This site is maintained by a brain injury survivor and not a medical professional. Although all efforts are made to ensure that content is factually accurate, the author is not responsible for how the information is used. If you have any concerns about your health, seek medical attention from NHS Direct, your GP or your local hospital, or in an emergency call 999.