Subjects selected for participation in this study have been diagnosed with Multiple Sclerosis (MS). Of the MS patients enrolled in the study, some have been treated with natalizumab or a different medication for MS, and others have not been treated at all. All MS patients enrolled have their blood tested for the presence of the JC virus. Those testing negative for the JC virus do not continue in the study. Those testing positive for the JC virus continue participating in the study, and provide a urine sample, blood sample, lumbar puncture, and a neurological exam. Approximately 65 people will take part in this study at Beth Israel Deaconess Medical Center.