Family Releases Cause of Kyle Busch's Death

Kyle Busch's family said Saturday that the two-time NASCAR Cup Series champion died of severe pneumonia that escalated into sepsis, causing fast-moving complications. Busch, 41, died Thursday after being hospitalized with what relatives initially described as a serious illness. In the statement, NBC News reports, his family said the infection progressed to sepsis. That's a medical emergency that occurs when the body has an extreme, overactive response to an infection, causing the immune system to damage its own tissues and organs, according to the Centers for Disease Control and Prevention.

The immune system then typically releases chemicals to fight off pathogens like bacteria, viruses or fungi, but with sepsis the response becomes excessive, per the AP. The results can cause widespread inflammation, form microscopic blood clots, and make blood vessels leak. Busch was thought to have had a sinus cold while racing on May 10. A statement from Kyle Busch Cos. said the family received the medical evaluation on Saturday, per ESPN. The Indianapolis 500 plans to honor Busch on Sunday by illuminating the scoring pylon on Lap 18, for the number on his car.