Progressive multifocal leukoencephalopathy (PML) kills by destroying the cells that insulate your brain’s wiring. A virus called JC virus infects and bursts open oligodendrocytes, the cells responsible for producing myelin, the protective coating around nerve fibers. Without myelin, nerve signals slow down, misfire, or stop entirely. The damage spreads across multiple brain regions, progressively stripping away the ability to think, move, see, and eventually sustain basic life functions. Most cases lead to coma or death despite treatment.
How the JC Virus Destroys Brain Cells
About half of all adults carry the JC virus without any symptoms. It typically lies dormant in the kidneys and bone marrow, held in check by a healthy immune system. When the immune system is severely weakened, whether by HIV/AIDS, certain immunosuppressive drugs, or conditions like leukemia, the virus reactivates and travels to the brain.
Once in the brain, JC virus targets oligodendrocytes with striking specificity. These cells wrap around nerve fibers and produce myelin, a fatty sheath that allows electrical signals to travel quickly between neurons. The virus hijacks the oligodendrocyte’s machinery to make copies of itself, filling the cell’s nucleus with new viral particles until the cell ruptures and dies. Under a microscope, infected oligodendrocytes appear swollen, their nuclei packed with dense clusters of virus. The surrounding astrocytes (support cells) take on bizarre, misshapen forms with abnormally large and multiple nuclei.
Each destroyed oligodendrocyte leaves a patch of exposed, uninsulated nerve fiber. Because the brain cannot regrow myelin once it’s lost to PML, the damage is permanent and cumulative. New virus released from each burst cell infects neighboring oligodendrocytes, and the zones of destruction expand outward.
Where the Damage Spreads
PML creates multiple patches of destruction scattered across the brain’s white matter, the deep tissue made up of myelinated nerve fibers connecting different brain regions. Lesions are typically asymmetric, appearing in different locations on each side of the brain, and they concentrate in the area beneath the cortex (the brain’s outer layer). On MRI, affected tissue has a characteristic “ground glass” appearance on certain imaging sequences, with lesions often larger than 3 centimeters. They tend to have a sharp border where they meet the cortex and a fuzzy, irregular edge where they bleed into deeper white matter.
The most commonly affected areas include the frontal, parietal, and temporal lobes. Lesions can also appear in the structures connecting the two brain hemispheres, the basal ganglia, and the thalamus. Less commonly, damage extends into the brainstem, which controls breathing, heart rate, swallowing, and consciousness. When lesions reach the brainstem or spread along major nerve pathways like the corticospinal tract (which carries movement commands from the brain to the body), the consequences are especially severe.
How Symptoms Progress
The symptoms of PML depend entirely on which parts of the brain are being destroyed, and they worsen over weeks to months as lesions expand. Common early signs include clumsiness or weakness on one side of the body, difficulty with balance and coordination, and trouble finding words or understanding speech. Vision loss affecting one side of the visual field is also frequent. Cognitive decline, from difficulty concentrating to confusion to severe dementia, often develops as more brain tissue is lost.
Because the damage is multifocal (hitting several areas at once), patients typically experience a combination of deficits rather than a single problem. Someone might simultaneously lose the ability to walk, read, and form coherent sentences. The progression is relentless. Over weeks, patients may lose the ability to swallow, increasing the risk of aspiration pneumonia (food or liquid entering the lungs). Eventually, the accumulating brain damage leads to a vegetative state or coma.
The most likely immediate cause of death is the combination of profound neurological shutdown and secondary complications. When the brain can no longer regulate basic functions like breathing and swallowing, patients become vulnerable to respiratory failure and severe infections. The brain damage itself can also be directly fatal when it reaches critical structures in the brainstem.
Why Treatment Often Fails
There is no antiviral drug that kills JC virus. The only strategy that sometimes works is restoring the patient’s immune system so it can fight the virus on its own. For people with HIV, this means starting or optimizing antiretroviral therapy. For patients on immunosuppressive medications like natalizumab (used for multiple sclerosis) or rituximab, it means stopping the drug and waiting for immune function to return.
This approach creates a dangerous paradox. When the immune system rebounds, it can trigger an aggressive inflammatory reaction in the brain called immune reconstitution inflammatory syndrome, or IRIS. The newly recovered immune cells, particularly a type called cytotoxic CD8 T cells, flood into the infected brain tissue and attack the virus, but in doing so they cause massive collateral damage to surrounding tissue. PML-IRIS can cause a sudden, dramatic worsening of neurological symptoms and carries significant risk of death on its own. Steroids are sometimes used to control the inflammation, and their use along with contrast-enhancing patterns on MRI scans has been associated with better survival odds, but the treatment is far from reliable.
Survival Rates and Timelines
PML carries one of the highest fatality rates of any brain infection, though the numbers vary significantly depending on the underlying cause of immune suppression. Before effective HIV treatment existed, the median survival after a PML diagnosis was just under five months. Modern antiretroviral therapy extended that to about 1.8 years, but the overall mortality rate in older studies still reached 74 to 78 percent.
More recent data paints a somewhat more nuanced picture. In HIV-positive patients, the one-year survival rate is roughly 78 percent overall, but it depends heavily on how damaged the immune system is at the time of diagnosis. Patients whose CD4 T cell counts (a key measure of immune health) are above 50 cells per microliter at diagnosis have an estimated one-year survival of 89 percent. For those with counts at or below 50, one-year survival drops to 59 percent. The best outcomes, around 95.5 percent one-year survival, occur in HIV patients who have higher CD4 counts and no other active opportunistic infections.
For people who develop PML from causes other than HIV, such as immunosuppressive medications or blood cancers, the prognosis is considerably worse. In one study, the one-year mortality rate in this non-HIV group was 60 percent, compared to about 26 percent in the HIV group. The overall mortality rate reached 80 percent for non-HIV patients versus roughly 41 percent for HIV patients. The median time from diagnosis to death in the non-HIV population was just two months, with a case-fatality rate as high as 90 percent in some reports.
The reason for this gap is straightforward. HIV patients have a clear path to immune restoration through antiretroviral therapy, giving the immune system a chance to suppress the virus. Patients on immunosuppressive drugs can stop those medications, but immune recovery is slower and less predictable. Patients with blood cancers or organ transplants often cannot safely restore full immune function at all, leaving the virus essentially unchecked.
How PML Is Confirmed
Diagnosis relies on a combination of brain imaging and spinal fluid testing. MRI is far more sensitive than CT scans for detecting PML lesions. The characteristic pattern includes bright patches on certain MRI sequences in the subcortical white matter, often without the contrast enhancement seen in many other brain infections. About 15 percent of HIV-related PML cases and 40 percent of cases linked to natalizumab do show contrast enhancement, which can complicate the diagnosis.
A spinal tap to test cerebrospinal fluid for JC virus DNA using a technique called PCR is the key confirmatory step. Most labs can detect the virus at levels above 200 copies per milliliter, though specialized labs like those at the NIH can detect as few as 10 copies per milliliter. When JC virus DNA is found in the spinal fluid alongside the right clinical symptoms and MRI findings, most experts consider the diagnosis confirmed without needing a brain biopsy.