New cases of a rare brain infection, progressive multifocal leukoencephalopathy (PML) , have been liked to the lymphoma and arthritis drug Rituxan and the multiple sclerosis (MS) drug Tysabri . The cases have regulators in the U.S. and Europe searching for ways to reduce the risks of patients developing the often fatal brain infection. Progressive multifocal leukoencephalopathy is a viral infection that causes inflammation at multiple locations in the brain, leading to potentially life-