Serena Williams was hospitalized earlier this week for a pulmonary embolism—aka a blood clot. Why is that scary? Blood clots, which commonly occur in the lower legs, can move to organs like the heart and brain (in this case it was her lungs) and cause life-threatening complications.

Thankfully, Williams sought medical help in time—shortness of breath and coughing up blood (eek!) are signs of a pulmonary embolism. The alarming thing is that, as an active, fit young woman, Williams suffered from this condition in the first place. Among smokers, pregnant women, and people on bed rest, one of the biggest groups at risk for blood clots are frequent fliers (that's why they remind you to move around the plane on long flights).

If you fly a lot or are worried you might also be at risk, invest in a pair of compression socks or tights to increase circulation— we like these ones from Futuro. And consult your doctor asap if you notice symptoms like unexplained swelling, bruising, tenderness, and noticeably warmer skin in one spot on your legs. Check out the American Heart Association for more info.