An Overview of John Cunningham Virus

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John Cunningham virus or JC virus is another name for human polyomavirus 2. JC virus is very common—between 70% to 90% of the world’s population would test positive for it.

Who Was John Cunningham?

Researchers have known about the JC virus since the 1960s when a healthcare provider found it in the brain of a cancer patient named John Cunningham.

When scientists figured out what type of virus it was, they renamed it polyomavirus 2—though many people still know it as JC virus today.

For most healthy people, JC virus will not cause problems. The virus becomes inactive (dormant), and most people will never get sick from it—or even know that they carry it. However, people with weak immune systems are at risk of developing a serious, potentially fatal, illness if they get JC virus.

This article will go over what JC virus is. You'll learn about the symptoms of JC virus, how it's diagnosed, and how JC virus is treated.

Doctor talking to woman patient in his office
Shannon Fagan / Getty Images

What Is JC Virus?

The JC virus is a type of polyomavirus, a family of viruses that primarily infect mammals and birds. This virus is very common around the world and often causes no symptoms. It's easily passed on from parents to children or among people in close contact.

JC virus is most often found in the tonsils, but can also be in the gastrointestinal tract or kidneys. Unlike other germs, the JC virus can cross the blood-brain barrier.

While researchers are not exactly sure how it works, they know that once JC virus gets into the brain it infects the cells that make the protective coating on nerves (myelin). Without the covering, nerves lose their protection (demyelination) and the brain gets damaged.

JC Virus and PML

When JC virus gets into the brain, the biggest concern is that a person will develop a rare and potentially fatal disease called progressive multifocal leukoencephalopathy (PML).

PML causes severe and fast-moving damage to the white matter in the brain. Of people diagnosed with PML, 30% to 50% die within the first few months. Many people who survive have a lasting neurological disability.

How Many People Get PML from JC Virus?

We do not know why some people develop PML from JC virus and others do not. There are likely many factors, such as genetics, environment, and lifestyle, that affect your risk. That said, you may not get PML even if you have more than one risk factor.

Symptoms of JC Virus

In healthy people, the JC virus does not cause symptoms because it is not active. However, if it gets reactivated in a person with lowered immunity, PML develops and damages different parts of the body—usually the brain.

The symptoms related to JC virus and PML depend on where the damage is but can include:

  • Weakness
  • Clumsiness
  • Having a hard time thinking
  • Trouble talking or being unable to speak
  • New problems with eyesight or loss of vision
  • Loss of feeling in or difficulty moving your limbs
  • Dementia-like symptoms and personality changes 
  • Seizures
  • Headaches (more common in people who have HIV/AIDS)

The symptoms of PML may not start all at the same time. While they can come on gradually, the symptoms tend to get worse quickly once they start. It usually does not take long before a person becomes so sick they cannot get out of bed.

PML can cause death as soon as a month after someone gets sick if they don't get treatment, but some people may live up to a year or rarely two. 

Some symptoms of PML can also occur in the conditions that make a person more likely to develop it, such as multiple sclerosis. Your provider will make sure that you understand your chances of developing PML and check your risk for it regularly. 

How Do You Get JC Virus?

Most people are exposed to it during childhood, particularly within their families. Research has shown that members of the same familiar tend to carry similar, if not the same, strains of JC virus. Parents passing it to their children is thought to be one of the most frequent ways that JC virus is spread. 

When JC virus is in the kidneys, it can come out with urine and spread to other people who come into contact with it. Most healthy people will not have any symptoms of an infection and probably won’t realize they’ve been exposed to JC virus.

Once the virus gets inside someone’s body, it stays there but isn’t doing anything that could cause illness (dormant). It’s only if something reactivates the JC virus that a person can get sick.

Risk Factors for JC Virus

Some factors increase the risk of JC virus becoming active again, including:

People with weak immune systems are more at risk for many types of infections, including one caused by JC virus. The risk of exposure can occur at any time because JC virus is so common.

If someone is at high risk for infection from JC virus, they will need to have repeated tests to check for it even if they tested negative before.

JC Virus: Does Having MS Increase Your Risk for PML?

Having a weak immune system or taking certain medications that quiet it can also increase a person's risk of getting PML from JC virus.

One condition that's linked to this risk is multiple sclerosis (MS), mostly because of the medications used to treat the condition.

People with MS already have areas of damage (lesions) in their nervous system, including in their brains. MS is usually treated with medications that try to stop the immune system from attacking the body and causing the lesions to develop. 

The Food and Drug Administration (FDA) has identified several drugs used to treat MS as having the potential to increase a person’s risk of PML, including:

Special drugs given to people who need organ transplants to prevent rejection may also increase the risk of PML, including:

  • Imuran (azathioprine)
  • CellCept (mycophenolate mofetil)

Other drugs used to treat autoimmune diseases have been associated with an increased risk of PML, including:

Your risk for PML will increase the longer you keep taking medications that are known to increase your risk. If your provider thinks your risk of developing PML is high, they may have you stop some of your medications or switch to another treatment.

How Is JC Virus Diagnosed?

There are a few ways providers can diagnose JC virus and PML, from blood tests to brain biopsies.

Blood Tests

Your provider can order a special blood test to find out if you have JC virus antibodies.

If the test shows the virus is in your body, it does not mean you will definitely get PML. Your provider will monitor you for signs of illness or any additional factors that could increase your risk, such as a new medication you're taking.

If the test says that you do not have JC virus, you will need to continue to be tested—at least every six months—because you can be infected by the JC virus at any time. 

As long as your test for JC virus stays negative, your risk of PML will be low—about one in 10,000 compared to one in 125 for people who test positive for JC virus and take medications that increase their risk.

Spinal Taps

If you are having symptoms that could be related to JC virus, your provider might want to do other tests, such as a spinal tap (lumbar puncture). If a regular spinal tap is normal, your provider can also do a special test called a PCR to look for DNA from the JC virus in your spinal fluid. 

Imaging tests like an MRI or a CT scan can help your provider see into your brain, kidneys, or other organs. If the JC virus is active, they might see lesions in places where it has caused damage. 

If you start showing signs of PML, your provider might need to take a sample of tissue from your brain (biopsy) to check for JC virus, but it's usually not needed to make the diagnosis.

Can JC Virus Be Treated?

There is no way to cure JC virus or PML if it develops. However, there are some ways that providers can try to treat it and give a person the best chance at recovery. 

If you take medication to suppress your immune system, such as steroids, your provider will want you to stop taking them. Depending on why you were taking these medications, you may need to be closely monitored by your medical team.

Stopping these drugs comes with risks. Your provider will compare these risks to the risk of getting PML.

Is There a Cure for JC Virus and PML?

There is no cure once the JC virus gets into the brain and causes PML. If the infection is caught soon enough, there are treatments that may lower the chances of long-term complications.

Will Stopping My Medications Cure JC Virus?

Sometimes, people with PML start to feel better once they stop these medications—though JC virus can keep doing damage. Any damage to the brain that has already happened may not heal.

People with conditions such as HIV/AIDS are more likely to have long-term complications from JC virus and PML and may have different symptoms, such as a headache.

Other people do not feel better when they stop taking medication and may even feel worse. This is because, without the drugs to calm down their immune system, the body mounts a major response against any virus—including the JC virus.

This response can make a person's symptoms more intense. If this happens to you, your provider may prescribe other medications or treatments to help you feel better and prevent complications such as brain swelling. 

If you have been taking medications such as Tysabri, your provider will want to do a procedure to help flush the drug out of your system as quickly as possible.

Plasma exchange helps clear the medicine from your body and allows your immune system to try to fight JC virus on its own. You may need to have more than one plasma exchange to make sure all the medication is out of your body. 


JC virus is very common and won't make most healthy people sick. However, there are people who are more likely to get sick if they are exposed to JC virus. Having JC virus in your body as well as other risk factors can increase your chances of getting a very serious brain condition called PML, but it does not mean that you will definitely get sick.

There is no cure for PML and many people who develop it will die soon after they get sick. That's why it is very important that people who are at risk are closely monitored by their healthcare team. When it's caught early, PML can be treated and some people do recover, though they may have long-term neurological symptoms.

The treatment for JC virus usually involves stopping any medications that can increase a person’s risk for PML, such as those used to treat multiple sclerosis (MS) and other autoimmune conditions. If you have a condition or take a medication that increases your risk for PML, your provider will regularly test you for JC virus antibodies.

6 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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By Abby Norman
Abby Norman is a freelance science writer and medical editor. She is also the author of "Ask Me About My Uterus: A Quest to Make Doctors Believe in Women's Pain."