Serena Williams had emergency surgery for a blood clot in her lungs according to an article titled "Serena Williams undergoes emergency surgery: report" by Reuters March 2, 2011. The medical term for a blood clot in the lungs is pulmonary embolism and is a serious condition.
NBC news reporter David Bloom died of a pulmonary embolism when he was embedded with United States fighting forces in Iraq during 2003. An article titled "NBC Newsman Dies In Iraq" by Dan Collins April 6, 2003 and published on the CBS News website details what happen with Bloom. It is thought that Bloom's condition may have been caused by enduring prolonged periods of immobility due to riding in cramped conditions while being embedded with the military. So why would a world class athlete like Serena Williams have such a condition. Serena has been struggling with a long standing foot injury that came from a cut from glass and that will probably be looked at as a contributing factor. An injury such as that could have complications that contributed to the far more serious condition.