From FDA reports: Tysabri and Progressive multifocal leukoencephalopathy (PML) - On Aug, 4, 2013: 90,168 people reported to have side effects when taking Tysabri. Among them, 2,582 people (2.86%) have Progressive Multifocal Leukoencephalopathy and among them, 665 people (0.74%) have Death. http://www.ehealthme.com/ds/tysabri/progressive+multifocal+leukoencephalopathy