A virus carried by nearly nine in 10 Americans could be involved in triggering multiple sclerosis (MS) relapses, according to new research.
Scientists identified a connection between Epstein-Barr virus (EBV) and MS, finding that signs of active EBV in the bloodstream may rise in the months leading up to a flare-up.
EBV belongs to the herpesvirus family and is best known for causing infectious mononucleosis, commonly called mono or the “kissing disease.” After infection, the virus remains in the body permanently. It is typically inactive, although stress or a weakened immune system can allow it to reactivate.
An estimated 300 million Americans have encountered or been infected with EBV by adulthood.
Earlier research has also suggested that infection with EBV may increase the risk of developing MS by more than threefold.
Around one million people in the US live with MS. The autoimmune disease develops when the immune system attacks the protective coating around nerves in the brain and spinal cord, disrupting communication between the brain and the rest of the body.
For the latest study, researchers compared blood samples from people with MS with samples collected from healthy participants.
The analysis showed heightened EBV activity inside immune cells as long as three months before patients developed relapse symptoms, including muscle weakness, impaired vision, numbness and extreme fatigue.
Researchers found a link between the Epstein-Barr virus (EBV) and MS, suggesting that EBV activity in the bloodstream may rise before an MS relapse (stock image)
Among patients who went on to experience a relapse, immune cells—especially B cells—underwent changes and appeared more potent before the flare-up than during the relapse itself.
Published in Nature Medicine, the research included blood samples from 114 people with MS and 21 healthy individuals.
Using molecular tools including single-cell RNA sequencing, the team examined which types of immune cells changed most significantly before an MS flare.
Single-cell RNA sequencing allows scientists to study thousands of blood cells individually rather than examining the sample as a single, mixed population.
B cells, a type of white blood cell capable of carrying dormant EBV, showed the most pronounced changes.
The findings indicated that these cells were shifting from a resting state into an activated one, pointing to an internal trigger for the immune response. That activation became stronger in the period before an MS relapse.
Before a flare-up, EBV-linked signals were tied to the activation of genes associated with greater susceptibility to MS. The same pattern was not observed to the same extent in people whose disease was in remission or in healthy participants.
The study does not yet show how MS relapses can be prevented, but it points toward possible treatments designed to target EBV and ease the disabling symptoms associated with flare-ups.
“These findings open a new avenue for targeted therapeutics,” senior author Dr. Tanuja Chitnis said in a press release.
Most people with MS are white women from northern Europe, Canada and the northern United States. The autoimmune condition affects about one million Americans
“Most current MS treatments work by broadly suppressing the immune system. Our results suggest that more precise strategies could be developed to 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.
