The JC polyomavirus (JCPyV) belongs to the family Polyomaviridae, genus Betapolyomavirus. These are small, non-enveloped, double-stranded DNA viruses. Approximately 60-80% of the population is carriers of JC polyomavirus. Various ways of spread are being studied. These viruses are associated with a large spectrum of neurological diseases, including progressive multifocal leukoencephalopathy, JC-associated meningitis and encephalitis, JCV granule cell neuronopathy and numerous central nervous system malignancies. In the majority of individuals, the virus remains latent in the kidney tissue. However, in certain subgroups, it leads to the development of severe and frequently disabling or fatal diseases. These subgroups are usually immunosuppressed, which causes the virus to undergo lytic rearrangement. Understanding these diseases and the mechanisms by which they occur is paramount for making accurate diagnoses. Studying these diseases is necessary to develop effective treatments and limit their harmful effects.