**Core Concept**
Progressive multifocal leukoencephalopathy (PML) is a rare and often fatal demyelinating disease of the central nervous system, characterized by the progressive destruction of the white matter in the brain. This condition is caused by a viral infection that targets the oligodendrocytes, the cells responsible for myelination.

**Why the Correct Answer is Right**
PML is caused by the JC virus (JCV), a type of polyomavirus that infects oligodendrocytes and leads to the destruction of the myelin sheath. The virus preferentially infects individuals with compromised immune systems, such as those with HIV/AIDS, lymphoma, or taking immunosuppressive therapy. The JCV infects the oligodendrocytes, leading to the production of defective myelin and the subsequent destruction of the white matter in the brain.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because PML is not caused by a bacterial infection, such as bacterial meningitis.
**Option B:** Incorrect because PML is not a result of autoimmune disease, such as multiple sclerosis, although both conditions affect the central nervous system.

**Clinical Pearl / High-Yield Fact**
The JC virus is a common virus that can remain dormant in the body for many years, and reactivation is often triggered by immunosuppression. Patients with PML typically present with a range of neurological symptoms, including weakness, vision loss, and cognitive decline.

**Correct Answer:** C. The JC virus.