PML is a rare, but often fatal, demyelinating disease of the central nervous system (CNS). PML is caused by lytic infection of oligodendrocytes and astrocytes resulting in multiple areas of demyelination in the CNS.

What are the odds of getting PML?

“Individuals who have been on the drug for two years, who have seen prior immunosuppressant therapy, who are JCV antibody positive—that group of individuals develops PML at rates of one in 50 to one in 100.” These levels are “even higher than those in the HIV population before the rise of antiretroviral medications,”

Does everyone have PML?

About 1 person out of every 200,000 will get PML. That works out to a total of about 4,000 people a year in the United States and Europe combined.

In general, PML has a mortality rate of 30-50 percent in the first few months following diagnosis but depends on the severity of the underlying disease and treatment received. Those who survive PML can be left with severe neurological disabilities.