PML: The Dark Side of Immunotherapy in Multiple Sclerosis.

Progressive multifocal leukoencephalopathy (PML) is a threat to patients with multiple sclerosis (MS) who undergo treatment with drugs that interfere with normal immune physiology. Here, we summarize PML incidence in MS, risk factors, surveillance strategies, and case definitions to inform neurologists and other clinicians treating patients with immune-mediated diseases.