When Derek Stefureac was diagnosed with multiple sclerosis, a chronic disease that affects the central nervous system, he was a smoker who never exercised.

Everything changed when he had an "attack" at work when he was 39: His body seized for about a minute, and Stefureac told CBS News that he "thought he was dying." After seeing multiple doctors, he was diagnosed with multiple sclerosis.

"It was a pretty scary diagnosis, and I wasn't even sure what it was, to be honest. I didn't know anyone who had it," Stefureac, now 51, said. "As I learned more, a doctor said, 'It's a progressive disease, it's incurable. We have some therapies to slow down the progression, but the best thing you can do is get healthy. A healthy body is the best tool.' So that scared me enough to quit smoking, and as part of quitting smoking, to help me out and get healthy, I just started jogging." 

Now, 13 years after his diagnosis and those initial jogging sessions, Stefureac has run 36 marathons — including one in Antarctica and one on Mount Everest. After completing Australia's Brisbane marathon earlier in June, he's now run a marathon on every continent. He's built a community of runners, connected with others with his condition and his doctor says he's even managed to reverse the progression of his multiple sclerosis. 

What is multiple sclerosis? 

Multiple sclerosis is a disease that affects the central nervous system, according to the Mayo Clinic's website. The immune system attacks the myelin, or protective sheath, that covers nerve fibers. That causes communication problems between the brain and the rest of the body, causing a wide range of symptoms including numbness and weakness in the body, an unsteady gait, blurry vision and more. Eventually, it can cause permanent damage or deterioration of the nerve fibers.

Multiple sclerosis is "an unpredictable disease," said Dr. Bruce Bebo, the executive vice president of research at the National MS Society, who is not involved in Stefureac's care.

For Stefureac, who also takes medication to manage his condition, the disease most prominently manifested as a dragging foot that he noticed when he began jogging. Dr. Le Hua, a neurologist overseeing Stefureac's treatment, said he also had neurological dysfunction and some numbness, weakness and tingling in his body. He also had spinal lesions, which are "associated with a higher risk of disability" from multiple sclerosis, she said. 

How does exercise impact multiple sclerosis?  

Bebo said that a growing body of evidence supports the importance of exercise and other healthy lifestyle choices in helping treat multiple sclerosis. Even if exercise isn't reversing the disease's progression, it can help limit co-morbidities like high blood pressure that can accelerate the progression of multiple sclerosis. Exercise can also help promote plasticity of the nervous system, which can improve function and compensate for damage caused by multiple sclerosis, he said. 

Cardiovascular training like running can be especially helpful for managing multiple sclerosis, Bebo said, but a person doesn't necessarily have to be running marathons to see the benefits. 

"There's pretty much something for everyone, no matter what their level of ability or disability is," Bebo said. 

Hua said that Stefureac's case is "really unique" because he has actually seen signs of "disability improvement," where some difficulties he initially faced have gotten better. Many people may see signs of disability slowing, she said, but an actual improvement in disease progression not something she or others in her field see often. 

"Derek actually looks a lot better now than he did when he was first diagnosed in terms of disability," Hua said.

Stefureac told CBS News that he "doesn't even remember" the last time he dealt with a symptom of multiple sclerosis. 

"I only think of MS when I have to refill my prescription or make an appointment," he said. 

After running a marathon on every continent, what's next? 

Stefureac has completed his goal of running a marathon on every continent, but there are still more extreme events he wants to compete in. He's looking at a marathon in the North Pole, and running one on Tanzania's Mount Kilimanjaro in February 2025. He also wants to participate in more intense events like a five-day race across the Sahara Desert and an Ironman Triathalon in Hawaii.

"When I started, the goal was to get myself in shape and slow this progression down, and it has worked so, so amazingly," Stefureac said, adding that he hopes his story can serve as an inspiration for other people dealing with multiple sclerosis or chronic health conditions. 

"It sounds insane, but for me, I'm grateful for the diagnosis. It really was an eye-opener, and it turned my life around. I don't think I'd be doing seven continents if I never had been diagnosed with MS," Stefureac said. "No one could ever know I have MS. People are shocked when I tell them. I'd like to be a good example of like, 'This could be you.'" 

In:
• Multiple Sclerosis
• Marathon