A human viral infection caused by the virus, tick-borne encephalitis virus. The first phase of infection is a flu-like illness with headache, fever and muscle aches. Most people recover without any other symptoms. About one-third of people will later develop complications including meningitis (inflammation of the membrane covering the brain), encephalitis (inflammation within the brain) and/or myelitis (inflammation of the spinal cord). Symptoms of this phase include a new headache, confusion and/or numbness and weakness in the legs. Patients become infected with the virus during a tick bite.