JC Virus: What It Is, How It Affects You, and What You Need to Know

When your immune system is strong, the JC virus, a common polyomavirus found in up to 80% of adults. Also known as John Cunningham virus, it usually stays quiet in your kidneys and bone marrow. But if your immunity drops—because of drugs, disease, or aging—it can wake up and attack your brain. That’s when things get serious.

This virus doesn’t cause symptoms in healthy people. But in those with weakened immune systems, it can lead to progressive multifocal leukoencephalopathy, a rare, often fatal brain disease. PML destroys the white matter in your brain, causing weakness, vision loss, speech problems, and confusion. It’s not contagious like the flu, but it’s deadly when it strikes. The biggest risk? People taking immunosuppressants, drugs that calm the immune system to treat autoimmune diseases like multiple sclerosis, Crohn’s, or lupus. Medications like natalizumab, rituximab, and dimethyl fumarate have been linked to PML cases. Not everyone on these drugs gets PML, but knowing the signs could save your life.

You might be wondering: if so many people carry the JC virus, why don’t more get sick? The answer lies in your immune system’s balance. Drugs that suppress it too much—especially over long periods—remove the guardrails keeping the virus in check. That’s why doctors test for JC virus antibodies before starting high-risk treatments. It’s not about fear; it’s about smart prevention. If you’re on one of these medications, watch for sudden changes: trouble walking, blurry vision, or slurred speech. Don’t wait. Tell your doctor immediately.

The posts below cover real-world connections between immune-suppressing drugs and hidden risks like the JC virus. You’ll find guides on how to talk to your doctor about medication safety, what to ask before starting a new treatment, and how to spot early signs of trouble. Whether you’re managing multiple sclerosis, rheumatoid arthritis, or another chronic condition, understanding how your meds interact with your body’s natural defenses isn’t optional—it’s essential.