Encephalitis is an acute inflammation of the brain. Usually the cause is a viral infection, but bacteria can also cause it. It can be mild or severe. Most cases are mild. Examples of viral infections that can cause encephalitis include herpes simplex virus (the virus that causes cold sores and genital herpes), varicella zoster virus (the chickenpox virus), mumps virus, measles virus and flu viruses. In the UK, the most common virus to cause encephalitis is herpes simplex virus. Most cases of encephalitis are caused by the virus directly infecting the brain. However, sometimes encephalitis can develop if your immune system tries to fight off a virus and, at the same time, attacks the nerves in your brain in error. This is known as post-infectious or autoimmune encephalitis. Rarely, this type of encephalitis can develop after an immunisation. Adult patients with encephalitis present with acute onset of fever, headache, confusion, and sometimes seizures. Younger chi...