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What Causes Multiple Sclerosis? 6 Risk Factors to Know

August 27, 2026

Multiple sclerosis, or MS, is a condition that affects the brain, spinal cord and optic nerves. And after a diagnosis, one of the first questions many people struggle with is “Why did this happen to me?”

Was it stress? Something you ate? A vitamin deficiency? A family gene you didn’t know about? For the roughly 1 million adults living with MS in the U.S., it’s normal to wonder what causes multiple sclerosis.

“MS is a complex condition, and there isn’t one single cause,” says Brian Wong, MD, a neurologist with Hartford HealthCare Ayer Neuroscience Institute and co-director of the Multiple Sclerosis Center in Southington. “We believe it develops in the setting of a combination of factors including genetic risks and environmental factors.”

So while there isn’t one clear cause, researchers do know that certain genetic, environmental and lifestyle factors can play a role. Here’s what we know about what causes MS and what might increase your risk.

First, who is more likely to develop MS?

MS can affect anyone, but researchers have identified several common patterns. It’s more common in women and typically develops in people in their 20s and 30s.

Other factors associated with a higher risk include:

• Vitamin D deficiency.
• Certain immune-related genetic factors.
• Having a close family member with MS.
• Having obesity during childhood or adolescence.

“Risk factors aren’t guarantees,” says Dr. Wong. “They help us see patterns and guide research, but they don’t tell us exactly who will or won’t get MS. This is why we see MS as a complex condition with several contributing factors, rather than a disease with one cause.”

These patterns help researchers better understand the possible causes of multiple sclerosis, but they also can’t predict exactly who will develop it.

> Related: 7 Symptoms That Might Mean You Have Multiple Sclerosis

1. An immune system misfire

MS is an autoimmune disease. This means the immune system, which usually protects you from infections, mistakenly attacks healthy tissue.

“In MS, the immune system targets myelin, the protective covering around nerves in the brain, spinal cord and optic nerves,” says Dr. Wong. “When that covering is damaged, nerve signals can slow down or become disrupted, which is what leads to many MS symptoms.”

These may include:

• Numbness or tingling
• Weakness
• Vision changes
• Balance or coordination problems
• Fatigue
• Trouble walking
• Bladder or bowel problems
• Cognitive changes

“We don’t fully understand why the immune system starts to target the nervous system,” says Dr. Wong. “But we know it’s not caused by anything a patient did. MS can unfortunately occur in a person without clear genetic or environmental risk factors.”

> Related: Why Christina Applegate May Have Missed Early Multiple Sclerosis Symptoms

2. Genetics

So, is MS hereditary? Not exactly.

“Genetics can play a role, but MS is not passed directly from parent to child like some other conditions,” says Dr. Wong. “Having a family history may raise your risk, but it’s just one part of the puzzle. Many people with MS have no family history at all.”

Here’s what we know:

• Having a close family member with MS can increase your risk.
• Certain genes are linked to how the immune system functions.
• Most people with MS don’t have a family history of the condition.

“Many people with MS are the first person in their family to be diagnosed,” says Dr. Wong. “That’s one reason we don’t look at MS as a purely inherited disease.”

> Related: 6 Early Symptoms of MS to Watch For

3. Certain viral infections

One of the most studied environmental links to MS is Epstein-Barr virus, a common virus that can cause mononucleosis.

But this connection is easy to misunderstand. Many people are exposed to Epstein-Barr virus and never develop MS, so researchers don’t see it as a simple cause-and-effect relationship.

“Epstein-Barr virus has been linked to a higher risk of MS, but almost all people are exposed to this virus eventually,” says Dr. Wong. “We believe viral infections may be triggers for the immune system to become overactive in people who are already at risk.”

4. Low vitamin D levels

Not getting enough vitamin D has been linked to MS risk, but that doesn’t mean it directly causes MS.

“Low vitamin D has been linked to a higher risk of MS, but we don’t see it as a direct cause by itself,” says Dr. Wong. “It’s one of several factors that may affect the immune system, but we know that vitamin D helps to regulate the immune system.”

Researchers have paid close attention to vitamin D because:

• MS is more common in areas with less sunlight exposure, such as geographic locations farther from the equator.
• Sunlight helps the body make vitamin D.
• Vitamin D plays a role in immune system health.

“Low vitamin D doesn’t mean you caused MS by missing time in the sun or not taking a supplement,” says Dr. Wong. “Still, we often check vitamin D levels in people with MS or those at higher risk. If your level is low, your doctor can help you decide if a supplement would be beneficial for you.”

> Related: Can You Take Vitamin D as a Sleep Aid?

5. Smoking

Smoking is one of the possible causes of multiple sclerosis that researchers are often asked about. It’s also a risk factor people can take action on moving forward.

“Smoking has been linked to a higher risk of MS and may also be associated with more active or severe disease,” says Dr. Wong. “We think tobacco smoke can affect the immune system due to the inflammatory nature of smoking.”

That doesn’t mean smoking caused your MS. Like the other factors here, it’s just one part of a much bigger picture.

But if you smoke, quitting can still be an important part of your care plan. Your doctor may recommend:

• Nicotine replacement therapy
• Prescription medication
• Counseling or coaching
• A structured quit plan
• Follow-up support to help you stay on track

“Quitting smoking is not always easy, and patients shouldn’t feel like they have to do it alone,” says Dr. Wong. “The goal is to offer support to help protect overall health and lower risks related to MS progression.”

> Related: Is Vaping Worse Than Smoking?

Can you prevent MS?

Because the causes of MS involve a complex mix of factors, there’s no guaranteed way to prevent it.

That can be frustrating to hear, but it can also help reframe the conversation. MS is complex, and for many people, the most helpful next step is not looking back, but forward.

“Patients often worry they could have prevented MS, but the truth is that this is a complex condition influenced by many factors beyond anyone’s control,” says Dr. Wong. “The goal is to focus on what we can do now. While there is no cure for MS yet, treatment has improved a lot, and medications are very effective at slowing or preventing disability. With early diagnosis, the right treatment and ongoing care, most patients do quite well over time.”