Progressive multifocal leukoencephalopathy (PML) is a rare and usually fatal
viral disease characterized by progressive damage (
-pathy) or
inflammation of the
white matter (
leuko-) of the
brain (
-encephalo-) at multiple locations (
multifocal). It is caused by the
JC virus, which is normally present and kept under control by the immune system. JC virus is harmless except in cases of weakened immune systems. In general, PML has a mortality rate of 30–50 percent in the first few months and those who survive can be left with varying degrees of neurological disabilities.