The sudden death of Kyle Busch has drawn attention to a rare but devastating medical progression: when pneumonia escalates into fatal sepsis.

An ER doctor spoke with Fox News Digital about how sepsis can trigger a rapid health decline.

"Sepsis is actually not a specific disease or diagnosis, but rather the syndrome that occurs when the body has certain abnormal findings and a presumed infection," said Dr. Kenneth J. Perry, a South Carolina-based emergency medicine physician.

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The markers of sepsis include elevated white blood cell counts, a high or low temperature, and elevated heart and respiratory rates, according to Perry. Because of this, a patient with pneumonia is often already technically septic by definition.

While many people assume a worsening infection means bacteria are multiplying uncontrollably, it often has more to do with the body’s internal environment.

"It is often not the bacteria itself that is causing the specific decline," Perry said. "In most cases, it is a cascade of inflammatory processes that are set in motion by the infection."

When this inflammation spirals out of control, the body moves from having a manageable infection into severe sepsis. This is when otherwise healthy people can rapidly deteriorate.

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"The concerning thing that can happen with any individual ... is that sepsis can then lead to low blood pressure, worsening vital signs and organ damage," Perry said.

"As multiple organs fail, it becomes very difficult for the medical team to treat and can sometimes lead ultimately to death."

It is very unlikely to have pneumonia and not have any symptoms, according to Perry. Early signs can mimic a severe flu, including fevers, chills, a productive cough, and chest or back pain in cases where the lung is infected.

When sepsis begins to take hold, time becomes the most critical factor. "We have known for a number of years that early antibiotic therapy is beneficial in the treatment of sepsis," Perry said.

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If you or a loved one are managing an infection at home, the doctor says the following red flags mean you should bypass the clinic and head straight to the emergency room.

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While cases like Busch's are tragic, Perry stressed that this shouldn't cause widespread panic. Most patients with pneumonia do very well with standard oral antibiotics.

The NASCAR star's rapid decline underscores the importance of medical vigilance and "having a primary care physician with whom you have a good relationship," according to the ER doctor.

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"Monitoring symptoms while having easy access to primary care is a very beneficial and appropriate plan for most patients," he added.