PML, the JC Virus, and Misconceptions

Last updated: April 2018

Many people with Multiple Sclerosis live with a pretty big fear of three letters: PML. The very mention of this acronym, which stands for Progressive Multifocal Leukoencephalopathy, can be terrifying to many people who take particular disease modifying medications to slow down the progression of MS. The source of this fear is pretty understandable: simply put, PML can kill you. It has killed people with MS in the past, which is a big reason why people live in fear of it. A lot of people seem to want to blame the medication when it comes to talking about PML, because I think that there are some misconceptions about it. I want to help clear up some of the concerns.

What is PML?

PML is a rare brain disease caused by being infected by the John Cunningham (JC) virus. It can also be caused by the re-activation of the JC virus (much like herpes or the chicken pox, when you have JC, it never really goes away, but can flare up). Most people can have the JC virus and never know about it. There aren’t any physical symptoms that you would notice. In fact, it’s been estimated that between 70-90% of the population actually has it. JC only really becomes a problem if you have a compromised or weakened immune system. When your immune system is comprised, the JC virus can’t be kept in check and will cross the blood-brain barrier (think of that as a filter in the body that blood has to go through before getting to the brain and spinal cord) into the brain. Once there, it attacks myelin, just like MS, but much more aggressively. It can also leave cells that will destroy brain tissue. That destructive process once the JC virus crosses the blood brain barrier is PML and it’s what can kill you.

This isn’t just a Tysabri problem

This is all a big deal for people with MS because of our immune systems. Cases of PML have been found in people that have been taking several of the medications used to treat MS. So far, cases have been found in people who have taken Tysabri, Gilenya, Tecfidera, and Ocrevus. Tysabri is often considered the big culprit, with even those cases of those taking Gilenya and Ocrevus having previously taken it. A lot of people want to push a lot of blame to Tysabri but I think there needs to be a bigger understanding of what’s going on here. Yes, there is a greater risk of PML when you have the JC virus and take Tysabri. That’s because Tysabri weakens and alters your immune system. Remember, with MS, it’s our own immune system that’s attacking our body. So with many MS drugs, not only Tysabri, you are at more of a risk of PML or any infections (You’ve probably seen other articles talking about how many of us get sick a lot of the time, it’s the same reason). Any compromised or weakened immune system increases your chance of getting PML and a large number of MS drugs alter or suppress the immune system (which, again, is their purpose). This is not solely a Tysabri/MS drug thing though. People who get chemotherapy are also highly at risk of getting PML if they have the JC virus. PML is also common in HIV-AIDS patients (that’s actually the most common instance where people die of it), again, because their immune system is compromised.

Fighting the body's immune system comes with risks

Tysabri is a big concern because it has the longest history of cases of PML (with two deaths, those others have gotten PML and not died), all before people were tested for the JC virus. This was early on when it was the latest drug that everyone wanted to be on, the Ocrevus of its day, if you will. The reason Tysabri is a risk is because it’s good at its job. MS is an autoimmune disease, we are literally fighting our own immune system to treat it. Tysabri helps us fight our immune system, it suppresses it to help limit the damage it can cause us. Suppressing the immune system will always put us at risk for other potential issues, after all, our bodies have this system for a reason, to help protect us. Sadly for us, it’s not working right.


Fortunately, we now have a great test that wasn’t around when people sadly passed away from PML with MS. When someone is on one of the major MS treatments that poses an increased risk, they will often have a blood test done every six months or so. Doctors can now determine not only if you are positive, but the level of antibodies in your blood. This produces an index value: the higher the number, the greater the chance of PML. This has been a major breakthrough because it has allowed some people to test positive but remain on their treatment.


Taking any of the the disease modifying medications is a big decision. However, I do feel that some people steer away from potential life changing treatments because of PML. There is a risk in anything, but for many, there are risks to not taking medication as well (I have witnessed this first hand, my grandfather deteriorated and passed away because he went untreated with MS). PML is a serious issue, one that should be discussed with your MS specialist after having the proper blood work done. However, it’s important to understand PML and make an educated decision. It’s important to realize that it’s not solely a Tysabri problem, that no one has died from it since proper testing was instituted, that there are also repercussions for not using an MS treatment, and people can and do become positive for the JC virus and still take one of these medications.

By providing your email address, you are agreeing to our privacy policy.

This article represents the opinions, thoughts, and experiences of the author; none of this content has been paid for by any advertiser. The team does not recommend or endorse any products or treatments discussed herein. Learn more about how we maintain editorial integrity here.

Join the conversation

Please read our rules before commenting.

Community Poll

Have you taken our 2023 In America Survey yet?