Need to Know: What Is the JC Virus?

Tamara Sellman avatar

by Tamara Sellman |

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Editor’s note: “Need to Know” is a series inspired by common forum questions and comments from readers. Have a comment or question about MS? Visit our forum. This week’s question is inspired by the forum topic “Does PML worry you?” from April 18.

The world of multiple sclerosis (MS) research is riddled with acronyms that can muddy our understanding of risk factors, side effects, and other treatment concerns.

One such concern is the development of progressive multifocal leukoencephalopathy or PML. To understand your risks for developing PML, you first must know what the JC virus is.

JC virus: A short history

The John Cunningham (JC) virus is a typically harmless polyomavirus carried by most humans. The virus was named after the patient in whom it was first identified in 1971.

Exposure to the JC virus usually occurs during childhood, but can happen at any age. When active, the virus presents as a minor illness, but it can be asymptomatic and may lie dormant in the body.

However, for people with immune system concerns, the presence of JC virus is problematic; it can quietly reactivate at any time and could — in a “perfect storm” — lead to PML.

People with compromised immune systems include those with MS using specific disease-modifying therapies (DMTs). When the JC virus is reactivated, it can lead to PML, a rare, life-threatening brain infection.

Do you have the JC virus?

A blood test is the only way to identify the presence of the virus. When someone says that they’re “JCV-positive,” or “JCV+,” they mean that they’ve had a blood test that was positive for anti-JCV antibodies.

Most people with MS are likely to have been tested for the JC virus early in their treatment. Knowing a patient’s JCV status helps their MS specialists to determine treatment options.

However, those who aren’t JCV-positive at diagnosis may need to be tested for it later when making changes to their medications.

What happens if you’re JCV-positive

It’s likely that you’re a JCV carrier. Don’t panic — this doesn’t mean that you’ll develop PML, nor does it indicate that you will be unable to treat your MS.

If you’re JCV-positive, three kinds of DMTs increase your risk for developing PML: Tysabri (natalizumab), Gilenya (fingolimod), and Tecfidera (dimethyl fumarate).

If you’re JCV-positive and not using any of these, there’s no risk of developing PML. A rare exception was documented in a new Ocrevus (ocrelizumab) user who previously took Tysabri.

If you’re both newly diagnosed and JCV-positive and haven’t started using any DMTs, you’ll likely start on a therapy that isn’t linked to PML.

If you’re currently using a DMT linked to PML and discover that you are JCV-positive, don’t worry. This doesn’t mean that you’ll develop PML. The risk of contracting it is very low. Your MS doctors will likely order regular blood draws as a part of an observation protocol.

In the above case, they are looking for a side effect of DMT usage known as lymphocytopenia, or lymphopenia.

Lymphocytopenia is a condition of having abnormally low levels of white blood cells, or lymphocytes, in the bloodstream. Lymphocytes work to target and destroy viruses and other toxins. Without enough of these hardworking blood cells, the body may be unable to fight off infections.

While infections such as urinary tract infections or pneumonia can be dangerous for people with compromised immune systems, they’re generally treatable, if caught early.

However, PML is considerably rarer, has a high fatality risk, and may develop rapidly. Effective treatment options are limited but include a cancer medication, pembrolizumab, which has shown promise in recent applications for treating PML in patients with MS. Research continues to search for markers and more effective treatments.

A diagnosis of lymphocytopenia in adults requires lymphocyte levels below 1,000 cells per microliter in a blood test.

Only after all of these factors come together: a JCV-positive determination, use of specific kinds of DMTs, and a diagnosis of lymphocytopenia, plus potential symptoms of PML — will a person with MS and their specialist need to assess their risks.

Do you know if you’re JCV-positive? Has your status influenced your decisions regarding MS treatment? Post your replies in the comments below or at the original “Does PML worry you?” forum entry.


Note: Multiple Sclerosis News Today is strictly a news and information website about the disease. It does not provide medical advice, diagnosis, or treatment. This content is not intended to be a substitute for professional medical advice, diagnosis, or treatment. Always seek the advice of your physician or other qualified health provider with any questions you may have regarding a medical condition. Never disregard professional medical advice or delay in seeking it because of something you have read on this website. The opinions expressed in this column are not those of Multiple Sclerosis News Today or its parent company, BioNews Services, and are intended to spark discussion about issues pertaining to multiple sclerosis.


Robby Teer avatar

Robby Teer

Oddly Tamara, I'll be taking my 76th straight infusion of Tysabri this month,have been, and still am JC Negative. THANKFULLY! All my doctor's informed me that while on my DMT (Tysabri) I would probably get sick easier and more often. However, I've not gotten any sicknesses more prevalently (if even at all) than I always have. I just turned 41 years old, and on average have MAYBE gotten sick once a year, with a cold, sniffles, runny nose, etc. Although, in my own case, it seems that I get sick even LESS since being diagnosed with MS in Sept. 2012. I stay at home with my 3 and 5 year old boys and had an extreme heat tolerance in the beginning. So bad, I couldn't even take a hot shower without my vision becoming distorted, losing my balance, and getting sick to my stomach. We purchased a new home 4/5 years ago, and I simply was NOT gonna have 2 little boys laid up in the house all day long. So, I went out in the dead heat of summer after we bought our house, I fell down, I bled, I puked. Then, I fell down, bled, and puked some more. But, I BEAT IT! I can now get out in the Alabama HEAT (humid like no other) and SWEAT with THE BEST!! This isn't to suggest everyone should do it. It's just that I've always been quite "HARD HEADED", as my Mom's always said. The Lord has to have been in my corner my whole life, because I've always "WON" at basically everything thrown my way, and have experienced few instances of "LOSING". My parents have always explained to people that "IF YOU WANT ROB TO DO SOMETHING, TELL HIM HE CAN'T, AND HE'LL BREAK BOTH HIS LEGS AND ARMS DOING IT", which I've always somewhat disagreed with, but completely agree with now at the ripe old age of 41. Tell me to do it, and I'm probably NOT GOING TO. Tell me NOT TO DO IT, and I almost always will. PML sounds awful, I definitely don't wanna contract it, and if I do, it's just another hurdle for me to beat. As long as I remain JC Negative, I'm told I'm in the clear. Is that in fact TRUE? That the ONLY WAY I could get PML while taking Tysabri is to be JC Positive?

Tamara Sellman avatar

Tamara Sellman

I cannot comment specifically on your case, only your MS specialist can clarify those kinds of specifics for you.

Generally, PML is extremely rare and, for those taking these DMTs, it requires the presence of both JCV+ status ***AND*** lymphocytopenia (lymphopenia). Think of it as the perfect storm.

Which is a rare situation, and the reason why people taking these DMTs must routinely perform labs to check their JCV status and their white blood cell counts. If you aren't, please do so.


Robert Birnbaum avatar

Robert Birnbaum

This article says that PML is possible only with three MS medications. It doesn't mention Ocrevus. Casn't this also lead to PML?

Danielle avatar


PML was found in 7 (last I read) patients taking Ocrevus, but all had previously taken a DMT known to carry a risk of PML - one on Gilenya and six on Tysabri. It's thought that the drug itself carries no PML risk, as no patients who hadn't been on a risky DMT beforehand have developed it. They think these cases were strictly carry-over from their previous meds.

Tamara Sellman avatar

Tamara Sellman

In that case ( the cause for PML was based on carry-over from Tysabri; it wasn't linked to use of Ocrevus.

Ra avatar


I was on Gilenya for two years and my white count was below 500 the entire time and most of the time in the 300-400 range. My neuro said that the Gilenya low count was not related to infections. Yes? No?

Tamara Sellman avatar

Tamara Sellman

Hi, Ra--
MS disease-modifying drugs work by modifying or modulating the immune response. Mostly that means, in simple terms, they shut down or weaken the immune system (MS is basically your immune system running in overdrive when the disease is active).

So I would say that the low count isn't related to infections, generally speaking. My WBCs (using Tecfidera) are similarly low. If your neuro isn't concerned, then you probably shouldn't be, especially if you are feeling okay otherwise.
But it's important to have this conversation with your own healthcare provider anyway (I'm no doctor) to ensure you understand how your medications work and what to expect from them.

Debbie avatar


So I’m JCV positive and my Dr is putting me on OCREVUS should I be concerned about getting PML please help I’m confused

Tamara Sellman avatar

Tamara Sellman

Hi Debbie
Share your concerns with your doctor. The risks are very tiny, and you're probably getting tests to watch for drops in white blood cells. You're probably fine but the best way to know for sure is to talk to your own neurologist. Good luck!

Jacques avatar


Hi Debbie, ask to see your latest blood tests. Ask if it was their family member..would they take a chance??? We have enough to deal with let alone the possibility of a treatment...supposing to help, that may or may not kill you. I dont gamble, especially with my life. Im also not a Dr. but have been through more than a half dozen Therapies

Jill avatar


I was on Tecfidera with my previous MD, only one blood test prior to start, no mention of JCV or PML. New and more competent MD just tested me & I am JCV+. I had no idea what this was! I was also misdiagnosed as RRMS, but am actually PPMS, so I would need Ocrevus now. Are there no cases of PML with Ocrevus after using Tecfidera?

Jill polsean avatar

Jill polsean

My former MD had me on Tecfidera for a year. He never mentioned JCV or PML, and I had only one blood test. I had never heard of JCV before yesterday when I got the results of blood test from my new MD. I am JCV+ and actually have PPMS, (thanks to new MD for the correct diagnosis) which means I now need Ocrevus. Are there any cases of PML with history of taking Tecfidera?

Jamie Creasman avatar

Jamie Creasman

I just got diagnosed with MS and also found out I’m JCV positive. I’ve started taking vumerity diroximel fumarate. I’m concerned about the PML is this one of the medicines that can cause this?

Lisa Piper avatar

Lisa Piper

For the 2nd time now I've tested positive for JCV?, I have monthly infusions of Tysabri with has now been put to 8 weekly , my neurologist is useless and won't even see me cos of covid rubbish, she's let me down massively

Elvina avatar


Hi, I tested positive for jcv and I am newly diagnosed with ms, I haven’t taken any dmt for ms before, and my doctor is putting me on rituximab and I am a little bit confused and worried.

Amy Stephen avatar

Amy Stephen

I recently started ocrevus and I was not tested for JCV prior to starting the infusion I am confused and now very concerned on how this is going to affect me. Prior to the infusions I was on Aubagio and my regular neurologist sent me to a ms specialist due to my ms getting worst so we switched meds. I just hope if I am by chance positive this is not going to induce PML.

Selma avatar


I am jc virus positive and I take aubagio 14 mg . I was wondering about covid vaccine and if it was safe with jc virus? My neurologist isn't helpful at all.

Stacey avatar


I was diagnosed with MS in 2018 and tested JCV positive I don't take any MS medicine give to many side effects the best thing I do is take Vitiamins ( Alpha Lipoic Acid) help support nerves and Vitamin D and B12 and Vitamin C and Zinc and Sea moss and Elderberry good for my nerve system my neurologist prescribed me with (Vumerity) I never took the medicine when I read the side effect of getting PML from the medicine I turn to Herbs like Dr.Sebi I'm not a doctor but eat healthy and exercise alot will help I don't have no MS flare ups

Lamarr avatar


I recently was at a routine urology appointment and a urine sample detected the JC virus. I have/had never heard of it, I've been doing a lot of reading, but I am a bit concerned to say the least after reading about PML and MS. I don't even know where I got this from, if I truly have it. I don't take any of those medications, but what do I do now?

Kathy avatar


Hi Stacey, can you advise how much Alpha Lipoic Acid you are taking daily?

Amanda avatar


I know this is a MS page, but has anyone been JCV+ and not been diagnosed with ms???

Denise avatar


Its interesting that your doctors are telling you the treatment your going on and my doctor just gives me a phamplet of the 25 medications and tells me to pick one with no direction what so ever.

Tania avatar


Hi I am new to MS. I have no chickenpox virus coming up in my bloods. I have been vaccinated for it 21 years ago 2 - doses but nothing coming up in bloods. Is it worth my while getting Vaccinated X2 4 weeks apart and starting Ocrevus in 12 weeks?
My JCV test is being sent now and have to wait 6 weeks for results. I am super scared of the JCV virus with treatment of Tysabri.


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