The JC virus is very common, and most people don’t know they carry it. But MS and other conditions affecting the immune system can activate the virus, causing a potentially life threatening infection.

The John Cunningham virus, known more commonly as the JC virus, is a very common virus in the United States. According to the World Journal of Neurosciences, between 70 and 90 percent of people in the world have the virus. The average person carrying the JC virus will never know and is unlikely to experience any side effects.

However, that’s not the case for a small percentage of individuals with multiple sclerosis (MS). The JC virus can be activated when a person’s immune system is compromised because of disease or immunosuppressive medication.

The virus can then be carried into the brain. It infects the white matter of the brain and attacks the cells responsible for making myelin, the protective coating that covers and protects nerve cells. This infection is called progressive multifocal leukoencephalopathy (PML). PML can be disabling, even fatal.

The JC virus often attacks when a person’s immune system is at its weakest. A weakened immune system can no longer fight off invading viruses. It’s the perfect opportunity for the JC virus to awaken, cross the blood-brain barrier, and begin attacking the brain. People with MS are at an increased risk for PML because their immune system is often compromised as a result of the condition.

Further compounding the problem, several medications used to treat the symptoms of MS can also compromise the immune system. Immunosuppressant medications can increase the likelihood that a person with MS will develop PML after exposure to the JC virus. These immunosuppressant medications may include:

In 2012, the U.S. Food and Drug Administration (FDA) approved the Stratify JCV Antibody ELISA test. One year later, a second-generation test was released to enhance the test’s accuracy.

This JC virus-detecting test can determine if a person has been exposed to the virus and if it’s present in their body. A positive test doesn’t mean a person with MS will develop PML, but only JCV-positive individuals can develop PML. Knowing that you’re JCV-positive alerts your doctor to watch for PML.

Even with a negative result, you’re not 100 percent safe. You can be infected with the JC virus at any point during your treatment.

If you begin taking medications as part of your treatment for MS, it’s important that you continue regular testing to see if you’ve been infected. Talk with your doctor about how often you should be tested for JC virus antibodies. If you become infected, regular testing will help you detect the infection faster. The sooner it’s detected, the sooner you can begin treatment.

Talk with your doctor about your risk for developing PML and how the medications you’re taking affect that risk. They may want to conduct an ELISA test out of an abundance of caution, especially if they plan to prescribe natalizumab (Tysabri) or dimethyl fumarate.

Natalizumab is often prescribed to people who haven’t responded well to other forms of MS treatment. According to an FDA Drug Safety Communication, studies show that people taking natalizumab are at an increased risk of developing PML compared to people with MS who are taking other disease-modifying medications. One such study was published in the New England Journal of Medicine in 2009.

If your doctor recommends you begin treatment with natalizumab, talk to them about having the ELISA blood test first. If your result comes back negative, you’re less likely to develop PML while on natalizumab. If your results come back positive, talk to your doctor about the danger of taking the medicine and the likelihood you’ll develop PML. A positive test may require that you and your doctor re-evaluate your treatment plan.

Doctors prescribe dimethyl fumarate to treat relapsing-remitting MS, including flare-ups or exacerbations of MS. According to Tecfidera’s manufacturers, the medicine cuts the risk for relapses in half when compared with people taking a placebo.

In 2014, the FDA released a safety announcement that a person treated with dimethyl fumarate developed PML. According to the New England Journal of Medicine, an additional case of dimethyl fumarate-related PML was reported in a woman treated for MS.

Like with natalizumab, doctors often recommend getting an ELISA blood test periodically while taking dimethyl fumarate.