What is a pulmonary embolism, the condition that led to Catherine O’Hara’s death?

By Katia Hetter, CNN (CNN) — A pulmonary embolism was the immediate cause of death for actor and comedian Catherine O’Hara. It’s a serious condition in which a blood clot lodges in the lungs. O’Hara’s sudden passing on January 30 at age 71 has drawn attention to a medical emergency that can unfold rapidly. What