PML is a rare inflammatory disorder that leads to loss of myelin in multiple areas within the white matter of the brain. It is a virus-induced disease seen in people with poor immune function (immunocompromised).

Symptoms and signs include loss of coordination, clumsiness, memory loss, progressively worsening weakness of the legs, and to a lesser extent, arms.

Other signs may include loss of language capability (aphasia), visual field defects, and headaches. The greatest risk is being immunosuppressed.

The causal virus, JC virus, is a common resident in most adults, but it triggers loss of myelin in those whose immune function is deficient. It occurs in 4% of adult individuals with AIDS.