The virus responsible for PML (progressive multifocal leukoencephalopathy), a rare brain disease that typically affects AIDS patients and other individuals with compromised immune systems, has been found to be reactivated in multiple-sclerosis patients being treated with natalizumab (Tysabri). The findings, led by scientists at Beth Israel Deaconess Medical Center (BIDMC), appear in The New England Journal of Medicine (NEJM).