I think the drug should be withdrawn from the market

I assume the "fatal brain disease" you are referring to is progressive multifocal leukoencephalopathy, a degeneritive and often fatal brain disorder caused by reactivation of the JC virus, a virus that is harmless unless a person is immunosuppressed, which can occur from chemotherapy, other immunosuppressent drugs such as the drug in question, and AIDS. Given the dismal prognosis for the disease (80% of patients are dead in six months) I feel that the drug should be withdrawn from the market. Now, as of 2010, at least 35 cases of the disease have occured in patients taking tysabri.