The John Cunningham virus (JCV) is a common virus that typically remains dormant in healthy individuals. It is part of the Polyomaviridae family and is primarily spread through respiratory secretions or urine. Most people are infected during childhood, but the virus usually does not cause any symptoms. In individuals with weakened immune systems, such as those with HIV/AIDS or undergoing immunosuppressive therapy, JCV can reactivate and lead to a serious condition called progressive multifocal leukoencephalopathy (PML). PML is a rare but often fatal brain infection that affects the white matter, leading to neurological symptoms.