Encephalitis is an inflammation of the brain, usually caused by a direct viral infection or a hypersensitivity reaction to a virus or foreign protein. The inflammation is a reaction of the body's immune system to infection or invasion. During the inflammation, the brain's tissues become swollen. The combination of the infection and the immune reaction to it can cause headache and a fever, as well as more severe symptoms in some cases.

There are more than a dozen viruses that can cause encephalitis, spread by either human-to-human contact or by animal bites. Encephalitis may occur with several common viral infections of childhood. Viruses and viral diseases that may cause encephalitis include:

• Chickenpox
• Measles
• Mumps
• Epstein-Barr virus (EBV)
• Cytomegalovirus infection
• HIV
• Herpes simplex
• Herpes zoster (shingles)
• Herpes B
• Polio
• Rabies
• Mosquito-borne viruses (arboviruses).

Mosquitoes spread viruses responsible for equine encephalitis (eastern and western types), St. Louis encephalitis, California encephalitis...