The JC virus (JCV) is a type of human polyomavirus and belongs to the family Papovaviridae. JCV is a double-stranded DNA virus, and is genetically similar to BK virus and SV40. The virus is very common in the general population and it is believed that most people acquire JCV in childhood or adolescence. Typically the infection is subclinical and no of consequence in children with healthy immune systems. The initial site of infection may be the tonsils or the gastrointestinal tract, and the virus then remains latent in the gastrointestinal tract. JCV can also infect the tubular epithelial cells in the kidneys, where it continues to reproduce, shedding virus particles in the urine. Also, JCV can cross the blood-brain barrier into the central nervous system. JCV is known to cause the usually fatal progressive multifocal leukoencephalopathy (PML) by destroying oligodendrocytes in the brain in immunodeficient or immunosuppressed individuals. However, it has not been established whether PML is the result of a primary infection with JCV in a person with impaired immunity or whether it follows reactivation of latent virus. JC virus is also the primary cause of nephropathy (kidney disease) in people who have received a kidney transplant and are on immunosuppressive therapy.