Encephalitis is inflammation of the brain, usually due to infection, according to Mayo Clinic. Encephalitis is a rare condition, and it can be either a primary or secondary infection.
Primary encephalitis occurs when an infectious agent or virus directly infects the brain, notes Mayo Clinic. Secondary encephalitis is a systemic response to infection elsewhere within the body, and it is generally characterized by the immune system attacking healthy brain cells. Although rare, live virus vaccinations can cause secondary encephalitis.
There are several common viral causes of encephalitis, including two types of herpes simplex virus, or HSV. Both the type of HSV that causes fever blisters and cold sores and the type that causes genital herpes can cause encephalitis, although the former type is less common than the latter. Enteroviruses, including coxsackievirus and poliovirus, can cause encephalitis. Mosquito-borne viruses, known as arboviruses, are another cause of encephalitis.
A virus carried by ticks, which is known as the Powassan virus, can also cause encephalitis, as can the rabies virus, although it is rare to see a case of encephalitis caused by rabies in the United States. Measles, German measles and mumps can also cause encephalitis, but due to the ready availability of vaccines against these common childhood infections, encephalitis caused by them is extremely rare, according to the Mayo Clinic.