Research published today in the New England Journal of Medicine has described how progressive multifocal leukoencephalopathy (PML) infection may occur in people taking Tysabri, a drug licensed for use in relapsing remitting multiple sclerosis (MS). PML is a serious and often fatal infection of the brain that is estimated to occur in approximately one in 1,000 people taking Tysabri over the course of 18 months.