What is PML?
PML stands for progressive multifocal leukoencephalopathy, which is a rare, progressive, infection of the brain that is caused by reactivation of a virus (the JC virus). Up to 80% of healthy adults have been infected with this virus at some point during childhood but the JC virus normally remains in the body in an inactive (latent) state. However, the JC virus can become active and cause illness when the body’s immune system (natural defenses against infection) is impaired by illness or medications. This is similar to what happens with the chickenpox virus that remains in the body and may become active later in life, causing shingles. In the medical literature, PML has been reported in patients with serious underlying diseases, such as cancers of the blood system (lymphoma, leukemia), HIV infection, or in patients who have had bone marrow, kidney, or heart transplants. In most reports, the patients were also receiving medications that impaired the body’s immune system. When the JC viru
