JCV infects brain cells called oligodendrocytes.
Oligodendrocytes wrap around nerves to produce myelin, a substance that protects nerves.
Without protection, nerves quickly become dysfunctional. Small lesions may also form. Inactivated JCV is harbored in the kidneys, but when the immune system malfunctions, JCV somehow moves to the bone marrow. B lymphocytes transport JCV to the brain from bone marrow. Symptoms usually progress rapidly.