Progressive Multifocal Leukoencephalopathy (PML) is a disease in which a virus ( polymaveris JC) attacks and deteriorates the myelin, which insulates the nerve cells. People with a higher risk to this disease are those with multiple sclerosis, HIV, and rheumatoid arthritis. Although experimental, extensive therapy with the drug enfuvirtide has been reported as a possible treatment. There are currently no drugs that can kill the virus without poisoning the patient. Unfortunately, PML has a morality rate of 30%-50% in the first few months.