Natural History Study of Progressive Multifocal Leukoencephalopathy (PML)

* PML Patients INCLUSION CRITERIA: 1. Suspected or confirmed PML 2. MRI compatible with PML 3. Able to participate in the studies and follow-up required by the protocol 4. At least 2 years old EXCLUSION CRITERIA: 1. Significant condition, which in the judgment of the principal investigator, would make participation in the diagnostic and research parts of evaluation impossible or risky 2. Medical contraindication to MRI (i.e., devices such as a cardiac pacemaker or infusion pump, other metallic implants, metallic foreign objects, body piercings that cannot be removed) 3. Pregnancy 4. Inability to provide informed consent, either directly or via legally authorized representative (LAR) 5. Unwillingness to consent for collection of biological samples or their cryopreservation Control Patients at Risk for PML INCLUSION CRITERIA: 1. Able to participate in the studies and follow-up required by the protocol 2. At least 2 years old 3. Impaired immune function from any cause and considered at risk for PML (i.e.-history of hematological malignancy, chronic inflammatory disease, history of treatment with immune suppressive or immunomodulatory therapies, primary or acquired immunodeficiency syndromes) EXCLUSION CRITERIA: 1. Significant condition, which in the judgment of the principal investigator, would make participation in the diagnostic and research parts of evaluation impossible or risky 2. Medical contraindication to MRI (i.e., devices such as a cardiac pacemaker or infusion pump, other metallic implants, metallic foreign objects, body piercings that cannot be removed) 3. Pregnancy 4. Inability to provide informed consent, either directly or via legally authorized representative (LAR) 5. Unwillingness to consent for collection of biological samples or their cryopreservation Healthy Volunteers INCLUSION CRITERIA: 1. Able to participate in the studies and follow-up required by the protocol 2. At least 18 years old EXCLUSION CRITERIA: 1. Impaired immune function (i.e.-history of hematological malignancy, chronic inflammatory disease, history of treatment with immune suppressive or immunomodulatory therapies, primary or acquired immunodeficiency syndromes) 2. Significant condition, which in the judgment of the principal investigator, would make participation in the diagnostic and research parts of evaluation impossible or risky 3. Medical contraindication to MRI (i.e., devices such as a cardiac pacemaker or infusion pump, other metallic implants, metallic foreign objects, body piercings that cannot be removed) 4. Pregnancy 5. Inability to provide informed consent, either directly or via legally authorized representative (LAR) 6. Unwillingness to consent for collection of biological samples or their cryopreservation