The JC virus (John Cunningham virus) PCR (quantitation) test detects the presence of the JC virus DNA in the cerebrospinal fluid (CSF) or blood. The JC virus is a common virus that infects most people during their lifetime but remains dormant in healthy people with intact immune systems. In people with weakened immune systems, such as those with HIV, cancer, or organ transplantation, the virus can reactivate and cause progressive multifocal leukoencephalopathy (PML), a rare and often fatal brain disorder.
