Why Do I Have MS? Understanding Multiple Sclerosis Risk Factors

By Beth W. Orenstein
Reviewed by Dana Cooper, M.D.
April 05, 2024

If you’ve been diagnosed with multiple sclerosis (MS), you might be wondering, “Why me?” The answer isn’t simple, but experts agree you’re not to blame.

That’s because there isn’t a single known cause of multiple sclerosis, though an increased risk of developing MS has been associated with several factors. “MS seems to arrive due to a confluence of circumstances—a perfect storm, if you will,” says Meghan Beier, Ph.D., a board-certified rehabilitation neuropsychologist at Johns Hopkins Medicine, in Baltimore.

And while some environmental and lifestyle factors can contribute to your overall risk for MS, rest assured it’s not something you brought on yourself with your behavior, Beier says.

“MS may have multifactorial causes, and it’s possible that these may vary from one person to another,” adds Daniel E. Smith, M.D., a board-certified neurologist at OhioHealth in Pickerington, Ohio. In other words, several MS risk factors, including those a person can't control, likely come together to cause MS.

Factors That Could Increase the Risk of MS

Here’s a look at what doctors and researchers believe may increase the risk of developing MS.

1. Genetics

This doesn’t mean you inherited your MS from Mom and Dad. Indeed, multiple sclerosis is not one of many diseases that are passed directly from one generation to the next, according to the National Multiple Sclerosis Society (NMSS). However, you can have a genetic predisposition, Beier says. Having a genetic predisposition for MS means having certain genes that may contribute to the development of the disease but that don’t cause MS directly.

“Researchers have identified subtle changes in more than 200 genes linked with either a higher or lower risk of developing MS,” says David Neriman Irani, M.D., a board-certified neurologist and director of the Multiple Sclerosis Clinic at University of Michigan Health in Ann Arbor.

Genes don’t tell the whole story, though, Irani says: “Even in identical twins, the risk of MS in one individual is only around 25% when the twin sibling has confirmed disease. Thus, we infer that some environmental exposure, perhaps that occurs relatively early in life, triggers the actual disease.”

2. Where You Live

By studying disease patterns in large groups of people, researchers have found that MS tends to occur more frequently in people who live in areas farther from the equator.

The thinking is that the farther from the equator you live, the less sunlight exposure you may have. The skin produces vitamin D when it is exposed to sunlight, so less sun exposure may mean less vitamin D. Some research suggests that low levels of vitamin D can be a risk factor for MS.

People who live in areas of the world that get more daily sun exposure tend to have higher levels of vitamin D as well as lower rates of MS. According to the National Institutes of Health, naturally produced vitamin D may support immune function and thus help protect against diseases like MS that are immune-mediated (caused by an overactive immune system).

3. Smoking

Evidence is growing to suggest that smoking may increase the risk not only of developing MS in the first place but also of having more severe and rapidly progressive disease if you do have MS.

“More data keep coming out that explores and seems to support the relationship,” Smith says.

Whether you smoke may be one MS risk factor that you can control, Beier notes. If you need help to quit smoking, ask. That goes for if you already have MS, too—quitting smoking can mean less disability and disease progression.

4. Obesity

Obesity has also been tied to an increased risk of developing MS, particularly in women and young girls. A 2021 study published in the Annals of Clinical and Translational Neurology found that people who are obese have higher levels of the hormone leptin. Leptin is released by fat tissue and promotes inflammation.

In the context of MS, the study found that leptin increased the activity of autoreactive T cells (cells that increase immune activity) and reduced the activity of regulatory T cells (cells that suppress immune activity). In other words, leptin seems to influence how the immune system launches an inflammatory attack on the central nervous system.

5. Exposure to Viruses

The list of possible MS risk factors also includes prior exposure to a common virus called Epstein-Barr virus (EBV), Smith says. EBV, also known as human herpesvirus 4, is one of the most common viruses in the world. Unfortunately, there aren’t good ways to prevent infection. It affects about 95% of adults at some point in their life.

EBV is the virus that causes the illness mononucleosis (also known as mono)—but many times, the infection doesn’t cause symptoms or only causes mild symptoms that can be mistaken for other illnesses.

A recent study found that EBV infection increased the risk of multiple sclerosis by 32 times in people who had other risk factors for MS. Other viruses and bacterial infections also may contribute to multiple sclerosis, and researchers are looking into their roles, according to the NMSS. These include the measles virus, canine distemper, human herpesvirus 6, and Chlamydia pneumoniae.

Unestablished Causes of MS

Some researchers have suggested that environmental allergies and exposure to pets, organic solvents, or heavy metals (including mercury in tooth fillings) could cause MS in some people. But there’s not enough evidence to support these claims, according to the NMSS.

Beier says that most factors that may lead to MS can’t be controlled or avoided, and it’s important for those who have been diagnosed with MS to understand that. At the same time, she says, it’s important for people with multiple sclerosis to work with their healthcare team to manage their MS so that their disease has a minimal impact on their quality of life.

Smith tells his patients that their MS is likely due to “the right genetics exposed to the right environmental triggers.”

Researchers must continue to explore possible causes of MS, Smith says: “If we can better understand [the answer to] this question, we can get closer to prevention.”

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