JC virus is a benign virus which infects approximately up to 90% of the normal adult population. However, it may be reactivated in people who have a decreased immune function as in HIV infection, cancer, chemotherapy, transplant recipients, or in MS patients treated with natalizumab (Tysabri). In these patients, JC virus can cause a severe brain disease called Progressive Multifocal Leukoencephalopathy (PML), for which there is no cure.