Is highly prevalent, and can be fatal in immunocompromised people The recent discovery of new neurological syndromes that result from neuronal infection with polyomavirus JC, also known as JC virus, and the presence of this virus in the grey matter are currently under debate.1 It has been suggested that JC virus is associated with cognitive decline, dementia, strokes, and brain tumours,2 and this hypothesis has been commented on in the BMJ .3 JC virus is a causal agent implicated in a rare but often fatal infection known as progressive multifocale leucoencephalopathy (PML).4 JC virus was first isolated in 1971 from the brain of a patient with PML; JC are the patient’s initials.5 PML is a demyelinating disease of the central nervous system that results from lytic JC virus infection of glial cells in immunosuppressed patients.1 It was first described as a complication of immune suppression 50 years ago and emerged as a major complication of HIV infection in the 1980s.6 PML has recently become topical because it is a side effect of some newly developed immunomodulatory drugs for autoimmune diseases, including natalizumab for multiple …