How pneumonia progresses to sepsis: Doctors explain after Kyle Busch's death

NASCAR star Kyle Busch’s cause of death was revealed Saturday as severe pneumonia that progressed into sepsis, according to a statement released by his family. The two-time Cup Series champion, 41, died on Thursday after a brief hospitalization with a "severe illness."

"The medical evaluation provided to the Busch Family concluded that severe pneumonia progressed into sepsis, resulting in rapid and overwhelming associated complications," read the statement, which was shared in a news release and reported by FOX Sports’ Bob Pockrass on Saturday.

Busch’s tragic case underscores the dangers pneumonia can pose, especially when left untreated or when symptoms quickly intensify.

NASCAR CHAMPION KYLE BUSCH’S CAUSE OF DEATH REVEALED BY FAMILY

Pneumonia is a respiratory infection that fill the lungs with fluid, making breathing difficult. It can be caused by bacteria, viruses or fungi, according to multiple medical sources.

Symptoms typically include cough, fever, chills, chest pain and shortness of breath.

Busch reportedly had a sinus infection that worsened prior to his death. "This upper respiratory sinus infection progressed to pneumonia," Fox News senior medical analyst Dr. Marc Siegel told Fox News Digital.

Bacterial pneumonia is normally more severe than viral pneumonia, according to Siegel.

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The doctor noted that the physical stress associated with racing simulators — which are designed to mimic the intense G-forces that drivers experience on the track — could potentially aggravate already inflamed lungs during recovery from pneumonia, though direct research on simulator-related effects is limited.

In severe cases, the infection in the lungs can spread into the bloodstream, triggering a widespread, life-threatening inflammatory response known as sepsis — particularly in vulnerable patients or when treatment is delayed, according to Siegel. 

Sepsis can quickly lead to tissue damage, organ failure and death if not treated right away.

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"The body reacts to this severe lung infection by making inflammatory chemicals – it’s the immune system revving up," Siegel said. "But as with a lot of things with the body, the immune system can hurt more than help."

As sepsis worsens, it can cause a drop in blood pressure and interfere with the delivery of oxygen to the body’s tissues, potentially leading to lactic acidosis — a dangerous buildup of lactic acid in the bloodstream.

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Organ failure is a serious risk, particularly affecting the kidneys, Siegel warned.

"The kidneys fail, toxins from the kidneys build up, blood pressure goes down, fever goes up, the lungs fail — something called ARDS," he said.

ARDS — acute respiratory distress syndrome — occurs when inflammation causes fluid to leak into the lungs, making it difficult for oxygen to reach the bloodstream.

Common warning signs of sepsis can include confusion, rapid breathing, extreme weakness, low blood pressure, fast heart rate, and bluish or mottled skin, per the CDC.

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Doctors emphasize that sepsis is a medical emergency because patients can deteriorate rapidly within hours.

Those at higher risk for severe pneumonia and sepsis include older adults and smokers, as well as those with chronic lung disease, diabetes, weakened immune systems or recent viral infections.