Multiple sclerosis develops when the immune system attacks the protective coating around nerve fibers in the brain and spinal cord

The cause of MS is not a single trigger but a combination of factors that researchers are still working to fully understand. What happens in MS is clear: your immune system mistakes the myelin sheath—the fatty insulation around nerve fibers—for a threat and attacks it. This damage disrupts the electrical signals that travel along nerves, which is why MS causes symptoms like weakness, vision problems, and fatigue. But why the immune system turns against your own tissue in the first place remains an open question.

Scientists have identified several pieces of this puzzle. Genetics play a role—MS runs in families and is more common in people with certain genetic markers. Environment matters too: where you live, infections you've had, vitamin D levels, and smoking history all influence risk. Neither genetics nor environment alone causes MS. Instead, researchers believe MS develops when a genetically susceptible person encounters one or more environmental triggers at a critical time.

Key Takeaways

  • MS occurs when the immune system attacks myelin, the protective coating around nerve fibers, but the reason this happens involves both genes and environment.
  • Having a genetic risk for MS does not mean you will develop it; most people with MS genes never get the disease.
  • Environmental factors linked to MS risk include viral infections (particularly Epstein-Barr virus), low vitamin D, smoking, and where you live.
  • Researchers have not identified a single cause or a reliable way to predict who will develop MS, which is why prevention is not yet possible.

The genetic component: inherited risk, not inherited disease

If a parent or sibling has MS, your risk of developing it is higher than the general population—roughly 2 to 3 percent, compared to 0.1 percent overall. But this is still a low absolute risk. Having the genes associated with MS is necessary but not sufficient; most people who carry these genes never develop the disease.

Researchers have identified over 200 genetic variations linked to MS risk, mostly in genes that control immune function. The strongest association is with the HLA gene region, which helps your immune system recognize foreign invaders. People with certain HLA variants are more likely to develop MS if they encounter the right environmental trigger. This explains why MS clusters in families but also why identical twins—who share 100 percent of their DNA—do not always both develop MS.

Viral infections and the immune system misdirection theory

The strongest environmental candidate for triggering MS is infection with Epstein-Barr virus (EBV), the virus that causes mononucleosis. Nearly all people with MS have been infected with EBV at some point, compared to about 95 percent of the general population. The timing matters: people who get EBV infection later in life (as teenagers or adults) have a higher MS risk than those infected in childhood, suggesting the age at which you encounter the virus influences whether your immune system learns to tolerate it or attack it.

The leading theory is molecular mimicry: EBV proteins resemble myelin proteins, so an immune response to the virus can accidentally cross-react with nerve tissue. Once this misdirected immune response begins, it can persist and worsen over time. Other infections, including human herpesvirus 6 and some bacterial infections, have also been studied as potential triggers, but the evidence is strongest for EBV.

Environmental factors that shift MS risk

Where you live influences MS risk in ways that suggest environmental exposure matters. MS is more common in higher latitudes—farther from the equator—in both the Northern and Southern hemispheres. One explanation is sunlight exposure and vitamin D production: people living farther from the equator get less sun and tend to have lower vitamin D levels. Low vitamin D is associated with higher MS risk, and some research suggests vitamin D helps regulate immune tolerance.

Smoking is another established risk factor. People who smoke have roughly double the MS risk of non-smokers, and the risk increases with the number of cigarettes smoked. Smoking may damage the barrier between the bloodstream and the nervous system, allowing immune cells to enter the brain more easily. Salt intake, obesity, and stress have also been studied, though the evidence is less conclusive than for smoking and vitamin D.

Migration studies show that people who move from a low-MS-risk region to a high-MS-risk region during childhood adopt the higher risk of their new location, suggesting environmental exposure during a critical developmental window matters. People who migrate as adults retain the risk of their birth region, indicating the relevant exposure likely occurs before adulthood.

Why the immune system attacks nerve tissue

MS is an autoimmune disease, meaning the immune system attacks the body's own cells. In MS, the target is myelin-producing cells in the central nervous system. Normally, the immune system learns to distinguish between self and non-self through a process called tolerance. Regulatory T cells and other immune mechanisms prevent the body from attacking its own tissue.

In MS, this tolerance breaks down. Researchers have found that people with MS have fewer or less functional regulatory T cells, and their immune cells are more reactive to myelin antigens. The question of why tolerance fails is not fully answered. It may involve a failure of the immune system to properly regulate itself, a viral infection that disrupts tolerance, or a combination of both. The fact that MS often begins with a relapse—a sudden flare of symptoms—suggests an acute trigger, but the underlying susceptibility develops over time.

What we do not yet know about MS causes

Despite decades of research, scientists cannot predict who will develop MS or prevent it from occurring. No single cause has been identified, and no test can determine whether someone will get MS based on genetics or environmental exposure alone. This is why MS is described as a disease of complex etiology—many factors contribute, but their interaction is not fully understood.

Researchers are investigating whether other infections, metabolic factors, or immune system abnormalities present at birth play a role. Some studies suggest the microbiome—the bacteria living in your gut—may influence MS risk, though this research is still early. The reason some people with genetic risk and environmental exposure develop MS while others do not remains unclear, and this uncertainty is why prevention strategies have not yet been developed.

Frequently Asked Questions

Can you inherit MS from a parent?

MS is not directly inherited like some genetic diseases. Having a parent with MS increases your risk, but most children of people with MS never develop it. The disease requires both genetic susceptibility and environmental triggers, and these do not always align even within families.

Does getting the Epstein-Barr virus mean you will develop MS?

No. Most people infected with EBV never develop MS. EBV appears to be a necessary factor in MS development for many people, but it is not sufficient on its own. The timing of infection, your genetic background, and other environmental factors all play a role.

Can you prevent MS if you have a family history?

There is no proven way to prevent MS. While some research suggests maintaining adequate vitamin D levels and avoiding smoking may reduce risk, these steps do not may provide prevention. If you have a family history of MS, discussing your individual risk with a neurologist can help you understand what to watch for.

Is MS caused by stress?

Stress does not cause MS, though some research suggests psychological stress may trigger relapses in people who already have the disease. MS develops from immune system dysfunction triggered by genetic and environmental factors that develop over time, not from stress alone.

Why do more women develop MS than men?

Women are about twice as likely to develop MS as men, but the reason is not fully understood. It may involve sex hormones, differences in immune response between men and women, or genetic factors linked to the X chromosome. Research into sex differences in MS is ongoing.