It's what many people want to know in light of the sudden death of Upper Darby High School athletic trainer, Brian Smart.
Smart suffered cardiac arrest shortly after mile eight on Sunday.
Dr. Neel Chokshi, a cardiologist at Penn Medicine, says a young adult athlete suffering sudden death is very rare, but not unheard of.
"When it happens like this, we think was there an irregular heart rhythm , that's usually what happens when something sudden happens and so the next step is what prompted that irregular rhythm," says Dr. Chokshi.
He says it could be several factors, including temperature or electrolyte imbalance.
It could also be a genetic predisposition for heart problems, scarring from an infection, or an anatomic anomaly.
"Probably what's least common is a blockage in one of the arteries, thats really rare in a young individual," says the doctor.
After Sunday's tragic death, Simon's Heart tweeted "our work is far from finished."
The non-profit foundation advocates for more thorough heart screenings for students, including an EKG.
Dr. Chokshi says some, but not all, conditions can be picked up with screening.
At the very least he says, athletes should have a good physical exam including family history.
Also talk to your healthcare provider if you have any symptoms such as passing out with exercise, chest pain, shortness of breath or any significant change.
"Suddenly you notice a decline in your performance without taking any time off or youre getting more short of breath or getting chest pain, that it just doesn't seem right to you well that might be a sign that something is going on," he notes.
Dr. Chokshi doesn't want to discourage anyone from exercising, but also reminds people to start slowly and always listen to your body. If you do have a family history of heart conditions or have experienced symptoms, talk with your healthcare provider.