Multiple sclerosis develops when the immune system turns against the protective coating around nerve fibers in the brain and spinal cord. No single cause explains every case, but researchers have identified a chain of factors that together raise the risk dramatically: a viral infection, genetic susceptibility, and environmental triggers that tip the immune system toward attacking its own tissue.
What Actually Happens in the Body
Nerve fibers in the brain and spinal cord are wrapped in a fatty insulating layer called myelin, which helps electrical signals travel quickly and efficiently. In MS, immune cells that normally fight infections cross into the central nervous system and begin destroying this myelin instead.
The process involves multiple parts of the immune system working in coordination. T cells, once activated, latch onto the walls of blood vessels in the brain and push through into surrounding tissue. Once there, they release chemical signals that recruit other immune cells, which then attack and strip away large chunks of the myelin sheath. Antibodies also target myelin directly, and the body’s inflammatory cascade amplifies the damage further. The result is scarred patches (lesions) scattered across the brain and spinal cord, disrupting signals that control movement, sensation, vision, and cognition.
Everyone has T cells capable of reacting to myelin proteins circulating in their blood. In most people, these cells stay dormant. The central question in MS research has been: what activates them?
The Epstein-Barr Virus Connection
The strongest single risk factor identified so far is infection with Epstein-Barr virus (EBV), the common virus that causes mono. A landmark study tracking over 10 million military service members found that people infected with EBV were 32 times more likely to develop MS than those who were never infected. That’s an enormous increase in risk for a single factor.
Critically, the study also tracked a blood marker of nerve damage and found that this marker only began rising after EBV infection, not before. This rules out the possibility that early, undetected MS somehow made people more vulnerable to the virus. The nerve degeneration started after infection and typically before MS was formally diagnosed.
Nearly 95% of adults worldwide carry EBV, yet only a fraction develop MS. So the virus appears necessary but not sufficient on its own. It likely sets the stage, perhaps by activating those dormant myelin-reactive immune cells or by triggering a case of mistaken identity where the immune system confuses viral proteins with myelin proteins.
Genetic Susceptibility
Your genes don’t cause MS directly, but they determine how vulnerable you are. The most significant genetic factor is a set of immune system genes called the HLA-DR15 haplotype, first linked to MS in the 1970s. This gene variant, found primarily in people of European descent, accounts for up to 60% of the total genetic risk. It shapes how your immune cells identify threats, which may explain why some people’s immune systems are more prone to mistaking myelin for something dangerous.
Beyond this major gene, large-scale genetic studies have identified over 200 additional gene variants that each contribute a small amount of risk. Most of these genes are involved in immune function. Having a first-degree relative with MS raises your risk meaningfully compared to the general population, but the majority of people with MS have no family history of the disease. Genes load the gun; something else pulls the trigger.
Vitamin D and Geography
MS follows a striking geographic pattern: it’s rarest near the equator and becomes increasingly common the farther you move toward the poles, in both hemispheres. This latitude gradient has been documented for decades and points to sunlight exposure, specifically the vitamin D your skin produces in response to UV light, as a protective factor.
People living at higher latitudes get less intense sunlight for much of the year, leading to lower vitamin D levels on average. Vitamin D plays a role in regulating the immune system, and low levels may remove a natural brake on the kind of immune overactivity that drives MS. Migration studies reinforce this: people who move from high-risk to low-risk regions before adolescence take on the lower risk of their new home, suggesting that sun exposure during childhood and teenage years matters most.
Smoking and Body Weight
Smoking increases MS risk by about 41% compared to never smoking. The mechanism likely involves chronic irritation of lung tissue, which activates immune cells in ways that can spill over into autoimmune responses. Smoking also worsens outcomes for people who already have MS, accelerating disability progression.
Obesity during adolescence is another established risk factor. Carrying excess body weight during the teenage years, a period when the immune system is still maturing, appears to shift immune regulation in ways that raise susceptibility. The combination of adolescent obesity and other risk factors like low vitamin D or EBV infection can compound the overall risk substantially.
Gut Bacteria May Play a Role
A growing body of research points to the gut microbiome as another piece of the puzzle. In a study of 81 pairs of identical twins where only one twin had MS, researchers found over 50 types of gut bacteria that differed between the affected and unaffected siblings. When bacteria from the small intestine of the twin with MS were transplanted into mice genetically prone to an MS-like disease, those mice developed the condition at substantially higher rates than mice receiving bacteria from the healthy twin.
The active bacteria belonged to a family called Lachnospiraceae. In mice that received them, immune cells shifted toward a more inflammatory profile, producing higher levels of inflammatory signaling molecules while showing reduced levels of regulatory cells that normally keep the immune system in check. The bacteria may either produce proteins that resemble myelin (confusing the immune system) or create a chemical environment in the gut that makes it easier for autoimmune T cells to activate. Since the gut houses a large portion of the body’s immune tissue, changes in its bacterial residents can ripple outward to affect immune behavior throughout the body.
How These Factors Work Together
MS isn’t caused by any one of these factors in isolation. The current understanding is that it develops through a sequence: a person inherits immune system genes that create vulnerability, gets infected with EBV at some point (usually in childhood or adolescence), and then environmental conditions like low vitamin D, smoking, or an unfavorable gut microbiome push the immune system past a tipping point. The immune cells that were always capable of reacting to myelin become activated and begin crossing into the central nervous system.
This explains why MS is more common in women (roughly three times as often as men), in people of Northern European descent, and in temperate climates. It also explains why the disease typically appears between ages 20 and 40: enough time has passed for the chain of events to unfold, but the immune system is still active enough to mount a sustained attack.
The Prodromal Phase
The process leading to MS doesn’t begin with the first obvious neurological symptom. Researchers now believe a prodromal phase begins at least 5 to 10 years before diagnosis. During this period, people are more likely to experience fatigue, depression, anxiety, pain, sleep problems, anemia, and bladder issues. They’re also more likely to seek healthcare in the five years before diagnosis, often for complaints that don’t yet point clearly to a neurological condition.
These early signs suggest that immune activity in the central nervous system begins well before it causes the kind of damage visible on an MRI or noticeable as numbness, vision problems, or difficulty walking. Recognizing this prodromal window is an active area of focus, since earlier treatment generally leads to better long-term outcomes.