Busch, who was 41 years old, succumbed on Friday (AEST) just a day after being admitted to hospital.
The two-time NASCAR Cup Series winner had been using the Chevrolet simulator at the GM Technical Centre in Charlotte on Thursday when he reportedly began coughing blood.
“The medical evaluation provided to the Busch Family concluded that severe pneumonia progressed into sepsis, resulting in rapid and overwhelming associated complications,” the Busch family said in a statement.
“The Family asks for continued understanding and privacy during this difficult time.”
Busch was known to have been battling an illness in recent weeks.
During the Cup Series race at Watkins Glen, he asked for medical assistance and a “shot” post-race.
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He went on to race a week later at Dover and won the NASCAR Craftsman Truck Series race.
That weekend, Busch said he had a substantial cough and sinus issues.
Busch’s tragic passing has been met with overwhelming grief in the NASCAR fraternity.
Joe Gibbs Racing driver Chase Briscoe said the star driver’s death should serve as a massive wake-up call.
“That’s something my wife has been really hard on me about just the last day or two. She’s like call (your friends) and just check on them,” Briscoe told media at Charlotte.
“Thats something I think we’ll all be more aware of. These last two days, we’ve all told these amazing stories of Kyle.
“For a lot of us, it was easy, but some guys have had that rivalry and have probably held a lot of that stuff in.
“We need to tell each other more often just how nice all of us are. At the end of the day, we’re all humans and all good dudes I feel like. So just trying to be better about that.
“Certainly, in the health standpoint, it’s a huge wake-up call,” he added.
“My wife just talks all the time about you guys just don’t get a break.
“It’s hard to take a week off. I think we’re all pretty hard-headed in the sense that we don’t want to go to the doctor and I’m guilty of that.
“Literally this December I had pneumonia, I didn’t go to the doctor for a long time. I finally went and found out I had it. It’s crazy just how fast this stuff can get you.
“Certainly I think it’s a wake-up call for all of us because it’s so easy to get caught up in ‘I’m going to go home, I don’t want to spend an hour at the doctor’s office. I want to be at home with my kids’.
“It’s hard with the schedule we live and I think it’s certainly a reminder for all of us that if you’re not feeling good you need to get checked out.”
Fellow NASCAR Cup Series driver Brad Keselowski said drivers took a laissez-faire approach to their health for fear of not racing.
“There’s no shortage of drivers that would love to take my seat or anybody else’s seat if we weren’t feeling well, and I think every driver feels that pressure,” Keselowski said.
“All athletes do. It’s not unique to NASCAR in that sense. We’re all thinking to ourselves, ‘I don’t wanna be replaced.’ … So you try to power through it the best you can.”
Busch has been replaced by Austin Hill for the Coca-Cola 600 on Monday (AEST) at Charlotte Motor Speedway.
The Richard Childress Racing car will carry #33 instead of the #8 that Busch ran.
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