Progressive multifocal leukoencephalopathy (PML) is a neurological disorder that damages the myelin that covers and protects nerves in the white matter of the brain. It is caused by the JC virus (JCV). By age 10, most people have been infected with this virus, but it rarely causes symptoms unless the immune system becomes severely weakened. The disease occurs, rarely, in organ transplant patients; people undergoing chronic corticosteroid or immunosuppressive therapy; and individuals with cancer, such as Hodgkin’s disease, lymphoma, and sarcoidosis. PML is most common among individuals with acquired immune deficiency syndrome (AIDS).

Currently, the best available therapy is reversal of the immune-deficient state. This can sometimes be accomplished by alteration of chemotherapy or immunosuppression. In the case of HIV-associated PML, immediately beginning anti-retroviral therapy will benefit most individuals.

The mortality rates for those with HIV-PML have fallen dramatically from approximately 90 percent to around 50 percent according to most reports. For non-AIDS individuals with PML, the prognosis remains grim; the disease usually lasts for months and 80 percent die within the first 6 months, although spontaneous improvement has been reported. Those who survive PML can be left with severe neurological disabilities.