John Cunningham Virus (JCV) is a type of virus that primarily infects humans and is part of the polyomavirus family. It is commonly found in the kidneys and can remain dormant in healthy individuals without causing any symptoms. However, in people with weakened immune systems, such as those with HIV/AIDS or undergoing immunosuppressive therapy, JCV can reactivate and lead to serious complications. One of the most severe conditions associated with JCV is progressive multifocal leukoencephalopathy (PML), a rare but often fatal brain infection. PML occurs when the virus attacks the protective covering of nerve cells, leading to neurological symptoms. Early diagnosis and management are crucial for individuals at risk of JCV-related complications.