The best known types are HIV-associated encephalopathy and dementia Subacute sclerosing panencephalitis (which occurs years after a measles infection and is thought to represent reactivation of the original infection; it is now rare in Western countries) Progressive multifocal leukoencephalopathy (which is caused by reactivation of JC virus; particularly in AIDS or immunosuppressed patients) Herpes simplex type 1 and herpes zoster encephalitis Immunologic reaction Encephalitis can occur as a secondary immunologic complication of certain viral infections or vaccinations.