Final answer:
The viral infection of the brain in immunocompromised individuals from the reactivation of the JC virus is known as Progressive Multifocal Leukoencephalopathy (PML).
Step-by-step explanation:
The blank in the question should be filled with Progressive Multifocal Leukoencephalopathy (PML), which is a rare and often fatal viral disease characterized by progressive damage or inflammation of the white matter of the brain at multiple locations. It is caused by reactivation of the JC virus, a common virus to which most people are exposed in childhood but which typically remains dormant. PML usually occurs in people with severe immune deficiency, such as those with AIDS, cancer, or transplant patients. The condition is diagnosed with the help of brain scans and laboratory tests.