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A rare virus linked to MS involving progressive brain damage.
There's no good way to sugar-coat this one—it's scary. Progressive multifocal leukoencephalopathy (PML) is a rare and usually fatal viral disease. Breaking down the term gives you a clear picture of the condition: it involves progressive damage (pathy) or inflammation of the white matter (leuko) of the brain (encephalo) at multiple locations (multifocal).