This document discusses progressive multifocal leukoencephalopathy (PML), an opportunistic infection caused by the John Cunningham virus (JCV) that leads to demyelination in the central nervous system. It highlights the epidemiology, clinical manifestations, diagnostic methods, and risk factors associated with PML, particularly in immunocompromised patients and those undergoing immunomodulatory therapies. It also presents data on the incidence of PML in patients treated with natalizumab and other therapies, addressing the relationships between these treatments and the development of PML.