We now have a cause of death for NASCAR driver Kyle Busch, who died Thursday at 41, three days before he was set to run the Coca-Cola 600 at Charlotte Motor Speedway.Related video above: Released 911 call sheds light on Kyle Busch’s medical emergency hours before deathHere is what we know:How did Kyle Busch die? Busch died from severe pneumonia, leading to sepsis, according to a statement from his family.In a news release, Dakota Hunter, vice president of Kyle Busch Companies, said the family received the medical evaluation Saturday."The medical evaluation provided to the Busch Family concluded that severe pneumonia progressed into sepsis, resulting in rapid and overwhelming associated complications," the family's statement read. "The Family asks for continued understanding and privacy during this difficult time."According to the Centers for Disease Control and Prevention, pneumonia is an infection of the lungs that affects millions of people every year. Symptoms include chest pain, cough, fever or chills, shortness of breath, nausea, vomiting and diarrhea.Sepsis is an extreme response to infection, where the infection triggers a chain reaction throughout the body and can lead to septic shock and multiple organs failing, according to the National Institute of General Medical Sciences. Symptoms include clammy or sweaty skin, confusion, extreme pain or discomfort, fever and shortness of breath.The majority of sepsis cases start before a patient goes to the hospital, the CDC reports, and 1 in 3 adults who die in a hospital had sepsis during their stay. The day before his death, Busch was testing in the Chevrolet racing simulator in Concord on Wednesday when he became unresponsive and was transported to a hospital in Charlotte, according to several people familiar with the situation who told The Associated Press on condition of anonymity because details have not been disclosed by Busch’s team or family.Had Kyle Busch been ill?Busch was thought to have had a sinus cold. While racing May 10 at Watkins Glen, Busch radioed his team requesting a “shot” from a doctor after he finished. Per the TV broadcast, Busch had been struggling with a sinus cold that was exacerbated by the intense G-forces and elevation changes at the New York road course.It's unclear if that issue had something to do with his death.Busch did race last weekend and won the Trucks Series race at Dover and finished 17th in the NASCAR All-Star race. That would be his final race.How good was Kyle Busch?Busch won 234 races across NASCAR's three national series — more than any driver in history. He won Cup season championships in 2015 and 2019 for Joe Gibbs Racing.His first championship was particularly impressive. Busch missed the first 11 Cup races of the 2015 season after injuring his foot following a crash in the O'Reilly Auto Parts series opener in Daytona, but came back to win five races to qualify for the playoffs before capturing the title at Homestead.Was the Coca-Cola 600 postponed or canceled?No. The 400-lap race will continue as planned Sunday night at the 1 1/2-mile track at Charlotte Motor Speedway.RCR announced that Austin Hill would replace Busch in the No. 8 Chevrolet Camaro ZL1.CMS officials said they plan to honor Busch “in some way” this weekend at the Coca-Cola 600, but details were still being ironed out.