Minor white matter abnormal signal on T2-weighted sequences reflecting vascular disease occurred in about 15% of both groups but extensive white matter abnormality was confined to seven non-prion cases (P<0.001); the diagnoses in these cases were SSPE, lymphoma, PML, CSF1R dementia, autoimmune encephalitis and encephalitis of unknown cause. This evidence concerns the gene CSF1R and viral encephalitis.