The JC virus, or John Cunningham virus, is a common virus that typically infects most people during childhood. It usually remains dormant in the body and does not cause any symptoms in healthy individuals. However, in people with weakened immune systems, such as those with HIV/AIDS or those undergoing certain treatments, the virus can reactivate and lead to serious neurological conditions. One of the most severe outcomes of JC virus reactivation is progressive multifocal leukoencephalopathy (PML), a rare but often fatal brain disease. PML damages the protective covering of nerve cells, leading to various neurological symptoms. Early detection and management are crucial for individuals at risk of JC virus-related complications.