What Is the JC Virus?

Medically Reviewed by Sabrina Felson, MD on July 31, 2020

The JC virus, or John Cunningham virus, is a common germ. More than half of all adults have been exposed to it. It doesn't cause problems for most people, but it can be dangerous if you have a weak immune system. There's no known way to keep yourself from getting it.

The virus was first discovered in 1971. A doctor found it in the brain of a man with Hodgkin's lymphoma and named the virus after them.

Experts don't know how it's spread, but it's thought that many people pick it up as kids through food or water that has the virus in it. It settles in your urinary tract, bone marrow, tonsils, or brain. It can stay there for years, and most people never know they have it.

Who Gets Sick?

In people with very weak immune systems, the virus can bring on a serious brain infection called progressive multifocal leukoencephalopathy (PML). PML damages the outer coating of your nerve cells. It can cause permanent disabilities and can even be deadly.

People with HIV/AIDS are most at risk for PML. Those with Hodgkin's disease, leukemia, or lymphoma are also at risk, as are people who take certain immune-suppressing drugs because of multiple sclerosis (MS), rheumatoid arthritis, or organ transplants.

Symptoms of PML

The symptoms usually come on fairly quickly and get worse over time. They can be different depending on the area of your brain that's affected, but the most common signs are:

  • Clumsiness
  • Weakness that gets worse
  • Changes in personality
  • Trouble speaking
  • Vision problems


Tests can detect if you carry the JC virus. Your doctor might look at a sample of your tissue under a microscope or check your blood for signs that your body is fighting the infection.

This is important to know in case you have a weakened immune system or if you take drugs that suppress your immune system.

If your doctor thinks you might have PML, you'll need an MRI (magnetic resonance imaging). This uses powerful magnets and radio waves to make detailed images and look for certain patterns in your brain tissue. Next, your doctor may want to do a spinal tap. They'll use a needle to take spinal fluid from your lower back. If the JC virus is in this fluid, you have PML. If not, you may need a biopsy of your brain tissue or other tests to find out for sure.

JC Virus, Multiple Sclerosis, and Crohn's Disease

PML has been linked to the drug natalizumab (Tysabri), used to treat MS and Crohn's disease. People with MS or Crohn's disease may be tested for the JC virus before they start this medication or others like it. If you're a carrier of the virus, you might still be able to take the drugs, but discuss the risks and benefits with your doctor.

People with MS and Crohn's disease who develop PML often have to stop taking the drugs. They also may have a procedure called plasma exchange to clear out the medication.

JC Virus and HIV/AIDS

Certain HIV drugs may make your immune system stronger and help prevent the JC virus from causing PML. These drugs have become more common, so the number of people with HIV/AIDS who have PML has been going down.

Treatment for PML

Scientists are studying several drugs for PML, but none has been approved by the FDA. However, treatments that make your immune system stronger may be used to help your body fight the infection. 

Show Sources


American Association for Clinical Chemistry: "JC Virus: What Is It, and How Should I Test for It?"

American Journal of Neuroradiology: "JC Virus Infection of the Brain."

Journal of Virology: Progressive multifocal leukoencephalopathy in a patient without apparent immunosuppression, published online December 2010.

Merck Manual: "Progressive Multifocal Leukoencephalopathy (PML)."

Multiple Sclerosis Trust: "JC virus."

National Institutes of Health, AIDS Info: "Guidelines for the Prevention and Treatment of Opportunistic Infections in HIV-Infected Adults and Adolescents."

National Institute of Neurological Disorders and Stroke: "NINDS Progressive Multifocal Leukoencephalopathy Information Page."

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