Christina Applegate, Selma Blair and John King have it. MS is more common than you think.

Celebrities getting sick with chronic conditions can cause those diseases to feel a lot more real.

Multiple sclerosis, commonly known simply as MS, is one such example. Actress Christina Applegate made headlines in 2021 when she announced on Twitter, now X, that she had the condition. Everclear lead singer Art Alexakis has been diagnosed with the condition as well; as has talk show host Montel Williams, Daytona 500 winner Trevor Bayne, actress Selma Blair, and CNN anchor John King, per WebMD.

Along with these celebrities, close to a million people in the U.S. have been diagnosed with the autoimmune disorder, according to the National Multiple Sclerosis Society.

Here's what MS is, what causes it and how you can expect the condition to be treated if it impacts you or someone you love.

What is MS?

Multiple sclerosis is a chronic disease that attacks the central nervous system, a system that includes the brain, spinal cord and a host of nerves. "It damages the protective fatty covering of nerves and disrupts communication between nerve cells," explains Dr. Roumen Balabanov, a neurologist at Northwestern Memorial Hospital in Chicago.

As this occurs, a person with MS may experience a range of symptoms such as impaired vision, memory lapses, mobility issues, muscle spasms, walking difficulties, imbalance, fatigue, mood swings and pain, numbness or tingling in their hands and feet.

Symptoms vary, however, depending on where the inflammation and damage is occurring at any one time. Flare-ups or relapses, as they are called, are times when MS symptoms are exacerbated and happen unexpectedly and usually last at least 24 hours.

"The unpredictable nature of the disease – when the next attack will occur, if or when the disability will progress – is debilitating for many people living with MS," says Julie Fiol, associate vice president of clinical innovation and strategy at the National Multiple Sclerosis Society. She adds that fears associated with losing independence, worsening cognitive abilities, remaining employed and the ability to participate in leisure activities often also hangs over people with MS.

In more advanced stages of the disorder, "many individuals with MS require supportive care," says Balabanov.

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What causes MS?

While the exact cause of MS remains unknown, scientists believe genetics and environmental factors both play a role. MS is not directly inherited, but "over 200 genes have been linked to the overall risk of developing MS," says Fiol.

As for environmental elements, "there are three well-established risk factors," says Balabanov. These include vitamin D deficiency, tobacco smoking and prior infection with Epstein-Barr (EBV) infection. "Additionally, obesity and an unhealthy lifestyle may contribute to disease susceptibility," he adds.

Exposure to high levels of air pollution or chemicals like solvents may also increase one's risk of developing the disorder.

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How is multiple sclerosis treated?

While there is no cure for MS, there are many medications that can modify the course of the disease and improve symptoms. "Scientists have made significant advancements over the past decade that have led to highly effective treatments," says Balabanov. These include monoclonal antibodies, "which can greatly reduce the risk of disease relapses and slow progression," he explains; as well as more aggressive treatments such as hematopoietic stem cell transplantation, "which offers the potential for long-term remission."

Fiol notes that these and other treatments "do not improve or repair damage that has been caused by MS, but they can reduce the risk of future damage and reduce the number of relapses someone has."

If diagnosed with multiple sclerosis, "individuals can expect a personalized treatment approach aimed at managing symptoms, reducing relapses and maintaining overall quality of life," says Balabanov. "With continued medical progress, the outlook for MS patients continues to improve."

This article originally appeared on USA TODAY: What is MS?