An Overview of John Cunningham Virus

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John Cunningham virus (JC virus) is another name for human polyomavirus 2. The virus is very common; between 70% to 90% of the world’s population would test positive for it. It doesn’t cause problems for most healthy people. The virus becomes inactive (dormant), meaning most people will never get sick or even know that they carry it. However, people who have weak immune systems are at risk for developing a serious, potentially fatal, illness from JC virus.

Researchers have known about the JC virus since the 1960s. A healthcare provider found the virus in the brain of a cancer patient named John Cunningham. When scientists figured out it was a type of polyomavirus, they renamed it (though many people still know it as JC virus). 

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


The JC virus is very common throughout the world. Most humans will be exposed to it during childhood, particularly within their family.

Research has indicated that members of the same familiar tend to carry similar, if not the same, strains of the virus. Parent-to-child transmission is likely one of the most frequent ways JC virus is spread. 

The virus is most often found in the tonsils, but sometimes it can be in the gastrointestinal tract or kidneys. Unlike other pathogens, the JC virus is able to cross the blood-brain barrier.

While researchers aren’t exactly sure how the mechanism works, they know that once the virus gets into the brain it infects the cells that produce myelin. Myelin is the protective coating on nerves. Without it, nerves become demyelinated and the brain is damaged.

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

PML causes severe and rapidly progressing damage to the brain’s white matter. Of people diagnosed with PML, 30% to 50% die within the first few months, and many people who survive have a lasting neurological disability.

In healthy people, the JC virus doesn’t cause symptoms because it isn’t active. If it becomes reactivated in someone with lowered immunity, PML develops and starts to damage different parts of the body—usually the brain.

A person’s symptoms will 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, and headaches (more common in people who have HIV/AIDS)

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

If it’s not treated PML can cause death as soon as a month after someone gets sick, though people may live up to a year (rarely two). 

Symptoms of PML are similar to the symptoms of the disorders that make a person more likely to develop it, such as multiple sclerosis (MS). Your healthcare provider will educate you about your chance of developing PML and assess your risk regularly. 


When the virus is in the kidneys, it can come out along with urine and be 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. 

Once the virus is inside someone’s body, it stays there, but it’s dormant, which means it isn’t doing anything that could cause illness. It’s only if something reactivates the virus that a person can become sick.

If the immune system is weak due to a disease like cancer or HIV/AIDS, a person is taking drugs to quiet (suppress) their immune system because they have an autoimmune disease, or they receive an organ transplant, there is more of a chance that the JC virus will be able to become active again. 

People who don’t have a strong immune system are more at risk for many different types of infections, including JC virus. Even if a person has not been exposed to the virus, there is a risk that exposure can occur at any time as the virus is so common.

If someone is at high risk for infection, they will need to have repeated tests to check for JC virus even if they tested negative for it before. While the complications from JC virus can be serious, a person has a better chance of getting better if the virus is caught early and treatment is started right away.

Risk Factors

Having certain diseases, including HIV/AIDS, cancer, and autoimmune diseases, or taking specific medications can increase your risk for developing PML from the JC virus. 

One condition frequently associated with this risk is multiple sclerosis (MS), mostly due to the medications used to treat it. People with MS already have areas of damage (called lesions) in their nervous system, including their brain. The condition is usually treated with medications that try to stop the immune system from attacking the body and causing these lesions to develop. 

The FDA has specifically 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 that can help prevent rejection may also increase the risk of PML, including:

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

  • Methotrexate
  • Corticosteroids
  • Cyclophosphamide

If you take these medications, your risk for PML will increase the longer you keep taking them. If your healthcare provider thinks your risk is high, they may have you stop the medications or switch to another type of treatment. 


Your healthcare 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 healthcare provider will monitor you for signs of illness or any additional factors that could increase your risk, such as a new medication.

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

As long as the test for JC virus remains negative, your risk of PML will be lowered—about one in 10,000 (compared to one in 125 for people who test positive for JC virus and are taking medications that have been linked to PML).

However, it’s important to know that healthcare providers still aren’t sure why some people develop PML from the JC virus and others do not. You may not develop PML even if you have more than one risk factor. There are likely other factors, such as your genetics, environment, and lifestyle, that also affect your risk. 

If you are having symptoms that could be related to the JC virus, your healthcare provider might want to do other tests, such as a spinal tap (lumbar puncture). While a regular spinal tap is usually normal, your healthcare provider can order 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 be used to look at your brain, kidneys, or other organs. If the JC virus is active, your healthcare provider may be able to see lesions where it has done damage in your body. 

If you begin to show signs of PML, your healthcare provider may need to take a sample of tissue from your brain (biopsy) to check for JC virus, though it’s usually not required to make the diagnosis.


There is no way to cure JC virus or PML if it develops, but there are some ways healthcare providers will try to treat it. If you get sick, your healthcare provider will take several steps to give you the best chance at recovery. 

If you take medication to suppress your immune system, such as steroids, your healthcare provider will start by having you stop taking them. Depending on the condition the drugs were treating, you may need to be closely monitored by your medical team.

Stopping these drugs comes with risks. Your healthcare provider will compare these risks to the risk of taking them if you could develop PML. 

There is no cure once the JC virus enters the brain and causes PML, but if the infection is caught soon enough there are treatments that may reduce the risk of longterm complications.

Sometimes people with PML do start to feel better once they stop these medications, though the JC virus can continue to do damage, and any damage to the brain that has already happened may not heal. People with certain conditions such as HIV/AIDS are more likely to experience long term complications 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, it will mount a major response against any virus—including the JC virus—which can make the symptoms more intense. If this happens, your healthcare provider may prescribe other kinds of medication or treatment to help you feel better and try to prevent complications such as brain swelling. 

If you have been taking medications such as Tysabri, your healthcare provider will want to perform a special procedure to help flush the drug out of your system as quickly as possible. Plasma exchange is a procedure that will help clear the medicine from your body and allow your immune system to try to fight the 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. 

A Word From Verywell

While having the JC virus in your body paired with other factors can increase your risk of PML, it does not mean you will definitely get sick. However, because there is no cure for PML and many people who develop it will die soon after they get sick, it is very important that people who are at risk are being closely monitored by their healthcare team. 

If caught early, PML can be treated and some people do recover, though they may have long term neurological symptoms. Treatment is mostly focused on stopping any medications that can increase a person’s risk for PML, such as drugs 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 healthcare provider will regularly test you for JC virus antibodies. Even if you do not have it at your first test, you can be infected with it at any time, so you will need to repeat testing as long as you have other risk factors for PML. 

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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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