These diseases include Progressive multifocal leukoencephalopathy (due to the JC [John Cunningham] virus, a polyomavirus) Subacute sclerosing panencephalitis (due to measles virus) Progressive rubella panencephalitis (due to rubella virus) Chronic viral infections are characterized by continuous, prolonged viral shedding; examples are congenital infection with rubella virus or with cytomegalovirus and persistent hepatitis B or C.