A virus carried by nearly 90 percent of Americans could help set the stage for multiple sclerosis relapses, according to new research.
The study adds to growing evidence connecting the Epstein-Barr virus, or EBV, with MS. Researchers suggest that heightened activity from the virus may be detectable in the blood in the months leading up to an MS flare-up.
EBV belongs to the herpesvirus family and is most widely associated with infectious mononucleosis, commonly known as mono or the “kissing disease.” After infection, the virus remains in the body permanently. It is generally inactive, though stress or a weakened immune system can allow it to reactivate.
An estimated 300 million American adults have been exposed to or infected with EBV.
Earlier research has also indicated that infection with EBV may raise a person’s risk of developing multiple sclerosis by more than threefold.
About one million Americans live with MS, an autoimmune disease in which the body’s immune system attacks the protective coating around nerves in the brain and spinal cord. The damage disrupts communication between the brain and the rest of the body.
For the latest study, scientists examined blood samples from people with MS alongside samples from healthy control participants.
Their findings showed greater EBV activity in immune cells from MS patients as much as three months before relapse symptoms emerged. Those symptoms can include muscle weakness, vision loss, numbness and intense fatigue.
Researchers found a link between the Epstein-Barr virus (EBV) and MS, showing that EBV activity may increase in the bloodstream before an MS flare up (stock image)
Among participants going through an MS relapse, the researchers also observed changes in immune cells—especially B cells. These cells appeared to mutate and become more potent before a flare-up than they were during the relapse itself.
The study, published in Nature Medicine, drew blood samples from 114 MS patients and 21 healthy subjects.
They then used molecular techniques, such as single-cell RNA sequencing, to identify which immune system cells changed the most before a flare up.
Single-cell RNA sequencing is when researchers analyze thousands of individual blood cells one-by-one, instead of looking at the sample as one whole mixture.
Researchers discovered that B cells, white blood cells that can harbor inactive EBV, showed the strongest changes.
They saw evidence that the cells were transitioning from their normal state to an activated state, suggesting that something in the body was triggering an immune response. This activation increased before an MS relapse.
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Before a relapse, EBV-related signals were associated with turning on genes that are known to increase a person’s susceptibility to MS. This pattern wasn’t seen in the same way when people were in remission or in healthy people.
The study doesn’t provide evidence on how to prevent an MS relapse, but opens doors for new treatment methods that target EBV to help with debilitating symptoms.
‘These findings open a new avenue for targeted therapeutics,’ senior author Dr Tanuja Chitnis said in a press release.
The vast majority of MS sufferers are white women in northern Europe, Canada and the northern half of the US. The autoimmune disorder affects roughly one million Americans
‘Currently, most MS treatments work by broadly suppressing the immune system. Our results suggest there’s an opportunity to be more precise and develop approaches that target EBV or the immune cells involved in relapse,’ Chitnis added.
The exact cause of MS remains unknown, but scientists believe a combination of genetics, environmental triggers, and viral infections — especially EBV — plays a key role.
Mono is extremely widespread, especially in teens and young adults, with about 500 out of every 100,000 people in the US getting it each year.
Although roughly 90 to 95 percent of people carry EBV, only about one in four who are exposed actually develop symptoms of mono.
EBV is primarily spread through saliva. In teens who are first exposed and develop mono, transmission typically occurs via kissing, earning the infection the nickname ‘the kissing disease.’
Mono typically causes extreme fatigue, a severe sore throat, fever and swollen lymph nodes in the neck and armpits.
Many people also develop swollen tonsils with white patches, headaches, body aches and an enlarged spleen, which can rupture if the person engages in contact sports or heavy lifting too soon.
While symptoms usually peak within two to four weeks, the fatigue can linger for months, especially in teenagers and young adults, who tend to experience more severe symptoms than younger children.
In an April 2026 study, published in Neurology Open Access, researchers tracked nearly 19,000 people and found that children and young adults who had lab-confirmed EBV followed by symptomatic mono were significantly more likely to develop MS later in life compared to those who never had EBV-positive mono.
Experts say the findings, published from a population-based study using medical records from the Mayo Clinic-led Rochester Epidemiology Project, underscore the urgent need for preventive strategies, including a potential EBV vaccine, to reduce the long-term toll of the devastating neurological disease.
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