JC virus: A virus that commonly causes infections of no consequence in children with a normal immune system, but which is responsible for an infection of the brain and spinal cord called progressive multifocal leukoencephalopathy (PML) in people with AIDS and other forms of immune system impairment. Abbreviated JVC. The JC virus is a human polyomavirus. It is a small virus with a closed circular genome consisting of double-stranded DNA. The letters JC are the initials of a patient with PML from whose brain the virus was first isolated.