Immunodeficiency or immunosuppression allows "Human polyomavirus 2" to reactivate. In the brain, it causes the often fatal progressive multifocal leukoencephalopathy, or PML, by destroying oligodendrocytes. Whether this represents the reactivation of "Human polyomavirus 2" within the CNS or seeding of newly reactivated "Human polyomavirus 2" via blood or lymphatics is unknown. Several studies since 2000 have suggested that the virus is also linked to colorectal cancer, as "Human polyomavirus 2" has been found in malignant colon tumors, but these findings are still controversial.