Serena Williams blood clot: What causes pulmonary embolism?

(CBS) Serena Williams is used to facing opponents on the court. But last week the tennis ace found herself facing a blood clot in the lungs, a potentially deadly condition doctors call pulmonary embolism.

"Last week, Serena suffered from a pulmonary embolism, which was discovered on her return to L.A.," Nicole Chabot, a representative for Williams, told People magazine. Chabot said Williams had been treated at Cedars-Sinai Medical Center in Los Angeles.

No word on what treatment Williams received, but pulmonary embolism can lead to death without immediate treatment with clot-busting medication, according to the Mayo Clinic website. In fact, about 12 percent of people who suffer pulmonary embolism die within a month of diagnosis, according to the American Heart Association.

Symptoms of pulmonary embolism include chest pain, cough - which can produce bloody spittle - and shortness of breath even in the absence of physical activity. Risk factors include family history, recent surgery, dehydration, and prolonged inactivity, often associated with bed rest or a long journey.

Then there are two other risk factors which certainly don't sound like Williams: Being old and being fat.

Williams is only 29, and anyone who's ever seen her swinging a racket knows she's hardly overweight.

Williams is being closely monitored by doctors, Chabot said.