JC virus is a common, harmless virus that silently infects an estimated 70% to 80% of the world’s population during childhood. For the vast majority of people, it causes no symptoms and remains dormant for life. It only becomes dangerous in people with severely weakened immune systems, where it can reactivate and cause a rare but serious brain disease called progressive multifocal leukoencephalopathy (PML).
How Most People Get Infected
JC virus (named after the initials of the patient it was first identified in, John Cunningham) belongs to a family of viruses called polyomaviruses. Most people pick up the virus during childhood or adolescence, and by adulthood, somewhere between 50% and 80% of the population tests positive for antibodies against it. The exact route of transmission isn’t fully established, but the virus is frequently shed in urine, suggesting person-to-person spread through contaminated water or close contact.
After the initial infection, which produces no noticeable illness, the virus settles into a dormant state in the kidneys, lymph nodes, and bone marrow. It can stay quiet in these tissues indefinitely. In healthy people, the immune system keeps it completely in check, and most carriers never know they have it.
When JC Virus Becomes Dangerous
The virus only poses a threat when the immune system is severely compromised. In that situation, JC virus can reactivate, enter the bloodstream, and travel to the brain. Once there, it infects and destroys oligodendrocytes, the cells responsible for producing the protective insulation (myelin) around nerve fibers. This destruction of myelin is what causes PML.
The people most at risk fall into two broad categories. The first is people living with HIV/AIDS: between 3% and 5% of all AIDS patients develop PML at some point during their illness, making it one of the most common settings for the disease. The second category is people taking certain immunosuppressive medications, particularly monoclonal antibody therapies that alter immune function.
Medications Linked to PML Risk
Several biologic drugs used to treat autoimmune conditions and cancers have been associated with JC virus reactivation:
- Natalizumab (Tysabri), used for multiple sclerosis and Crohn’s disease, carries the most well-documented risk. It works by preventing immune cells from crossing into the brain, which inadvertently removes the surveillance that keeps JC virus suppressed there.
- Rituximab (Rituxan), used for blood cancers and rheumatoid arthritis, targets a specific type of immune cell and has been linked to PML cases.
- Efalizumab (Raptiva), once used for psoriasis, was withdrawn from the market partly due to PML risk.
- Alemtuzumab and daclizumab, used for leukemia and multiple sclerosis respectively, have also been associated with cases.
How PML Risk Is Monitored
For people taking natalizumab, doctors use a blood test that measures the level of antibodies against JC virus, expressed as an “antibody index.” This number helps estimate PML risk. Patients who test negative for JC virus antibodies have the lowest risk. Among those who test positive, the index value matters considerably.
Patients with an antibody index of 0.9 or below who have no history of other immunosuppressive drugs have an estimated PML risk of about 0.1 per 1,000 in the first two years of treatment. That risk climbs with a higher index and longer treatment duration. Patients with an index above 1.5 face a substantially higher risk: roughly 1 per 1,000 in the first two years, jumping to about 8 per 1,000 after four or more years on the drug. These numbers help guide decisions about whether to continue, switch, or stop therapy.
Symptoms of PML
PML tends to develop gradually over days to weeks. The most prominent early signs are progressive weakness on one side of the body, clumsiness, and visual disturbances. Speech difficulties and personality changes also occur. Because the virus damages myelin in scattered patches across the brain, the specific symptoms depend on which areas are affected. The disease can mimic a stroke or a flare of multiple sclerosis, which sometimes delays diagnosis.
Diagnosis relies on a combination of brain imaging (MRI shows characteristic white matter lesions), neurological examination, and detection of JC virus DNA in cerebrospinal fluid through a spinal tap. Newer ultrasensitive testing methods can detect the virus in spinal fluid with about 85% sensitivity on the first test, rising to 95% with follow-up testing, and a specificity of 100%, meaning a positive result is highly reliable.
Treatment and Outlook
There is no antiviral drug that directly kills JC virus. The primary strategy is to restore the immune system so the body can fight the virus on its own. For people with HIV, this means starting or optimizing antiretroviral therapy immediately to suppress the virus and rebuild immune function. For people on immunosuppressive medications, the offending drug is stopped to allow the immune system to recover.
This immune recovery can itself create a complication. As the immune system rebounds, it may mount an aggressive inflammatory response against the JC virus already in the brain, a phenomenon called immune reconstitution inflammatory syndrome (IRIS). IRIS can temporarily worsen neurological symptoms and brain swelling, and it sometimes requires treatment with corticosteroids to control the inflammation.
The prognosis for PML remains sobering. In people with HIV who receive effective antiretroviral therapy, median survival has improved from about five months to roughly four years, and about a quarter of HIV-related PML patients show some neurological improvement over time. The outlook is considerably worse for non-HIV patients: median survival in one study was just 184 days, with a one-year mortality rate of 60%. Among survivors, severe disability is common. About 85% of non-HIV PML survivors and 59% of HIV-related survivors are unable to walk without assistance at their last follow-up. Overall, PML remains one of the most serious opportunistic brain infections, and none of the patients who improved in long-term studies were from the non-HIV group.
Living With JC Virus as a Carrier
If you’ve tested positive for JC virus antibodies, that result alone is not a cause for alarm. Most of the adult population carries this virus without consequence. The antibody test becomes relevant only in specific medical situations, particularly if you’re being considered for natalizumab or another high-risk immunosuppressive therapy. In that context, your antibody index, treatment duration, and history of prior immunosuppression all factor into a risk calculation that helps you and your doctor weigh the benefits of the medication against the small but real possibility of PML.
For people not on immunosuppressive therapy and without conditions like advanced HIV, JC virus requires no monitoring, no treatment, and no lifestyle changes. It is, for the overwhelming majority of carriers, a virus you’ll never notice.