The JC virus belongs to the polyoma virus group of the papovavirus family, which are double-stranded DNA viruses without an envelope. BK virus is also in the polyoma virus group. It appears that infection by both viruses occurs during childhood or adolescence, with about 50% of the population demonstrating antibody before adulthood, rising later to 80%-90%. The JC virus localizes to and remains latent in the kidney, from whence it occasionally may reactivate. If a patient becomes immunosuppressed, especially during AIDS, reactivated JC virus can infect lymphocytes, be carried to the brain, infect oligodendroglia glial cells, and produce a demyelinating disease called progressive multifocal leukoencephalopathy. This occurs in about 4% of patients with AIDS. Diagnosis is made through biopsy using immunologic stains containing antibody against polyomavirus. One report applied a homemade nucleic acid probe with PCR amplification to urine of JC virus patients and obtained an excellent detection rate.