The Risk of PML is greatest in the setting of a positive
J-lymphocytesC virus, greater than 24 months of
Tysabri, and concurrent use of other DMD's or steroids. Those are old recommendations, and new info will change them in the (near?) future.
PML is caused by the J-C Virus. A little more than half of all people have been infected with the JC virus. It generally lives asymptomatically in the brain unless something has occurred to cause the immunity of the brain to be compromised.