As of September 2013, 400 MS patients in the world, who have been treated with natalizumab, have developed PML. The risk of PML is approximately 5 patients in 1000 after 24 months on the drug. Researchers do not know exactly in which cells of the body the virus lives but it has been isolated from the blood, urine, cerebrospinal fluid (CSF), and from the brains of patients with immunosuppression.