Progressive multifocal
leukoencephalopathy (PML)

June 2007     View PDF     En español

PML is a rare AIDS-related condition of the brain, caused by the JC virus. World wide, more than 4 in 5 adults are exposed to this virus, but it only causes disease in people with weakened immune systems. Before AIDS, PML was rarely seen except in people with advanced cancer or bone marrow transplants.

Today, most PML cases occur in people living with HIV, mainly in those with advanced disease and very low CD4+ cell counts. However, it occasionally appears in people with CD4+ cell counts up to 500. Overall, about 1 in 20 people with AIDS will develop PML. It is unclear why, if most people with HIV also have the JC virus, the rate of PML remains so low.

Because it is so rare and because it affects the brain (an organ that is difficult to study), its diagnosis and treatment are poorly understood. In 2005 and 2006, several people with inflammatory diseases were treated with immune therapy (such as natalizumab) to block inflammatory cells and developed PML. This led to renewed interest in researching this rare disease. As PML becomes better understood, some debate may begin on changing the name of the condition to JC Virus Encephalopathy.
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