When news broke that NASCAR star Kyle Busch had died at the age of 41, fans were stunned.

Busch, who has won more NASCAR races in his two-decade career than any driver in the sport’s history, was not just healthy, he was a champion in a physically demanding sport.

After facing pneumonia in the weeks leading up to his death, Busch developed sepsis, an extreme reaction by the immune system to an infection that can cause organ and tissue damage. Sepsis kills 350,000 adults in the United States each year—one out of every three adults who dies in a hospital had sepsis during their stay, according to the Centers for Disease Control and Prevention.

Especially among younger people, the onset can be sudden and the symptoms hard to recognize, says Dr. Todd Rice, professor of medicine and director of the intensive care unit at Vanderbilt University Medical Center. Dr. Rice, who has not treated Busch and is not connected to his case, explains how an illness like pneumonia can cause sepsis, and why the condition can be so deadly, even for young, healthy people like Busch, who shared two young children with his wife Samantha Busch —son Brexton Locke, 11, and daughter Lennix Key, 4.

Pneumonia isn’t an uncommon disease. How can it turn deadly for someone like Kyle Busch?

“Pneumonia is one of the top causes of death for older people. Young people get pneumonia but usually they tolerate it pretty well. They get some chest pain, fever, shortness of breath and that prompts them to go see a doctor and they get some antibiotics and they do reasonably well. But there are cases where with an infection, the body has a response to control the inflammation so it doesn’t get widespread. With sepsis, it kind of gets out of control. Sepsis is sort of an imbalance where you get this inflammatory response throughout the body and that causes other parts of the body to not work very well. Your blood pressure may be low, your kidneys may not work very well, your brain may not work very well. It’s somewhat rare that we’ll see a young person that comes in with an infection like pneumonia that will have bad sepsis and will end up dying from it, but it’s certainly not unheard of.”

Why might an infection like pneumonia turn into sepsis?

“It’s a question we don’t know the answer to entirely. Why do some people have this over-exuberant, uncontrolled inflammatory response that affects other parts of the body and other patients don’t? There are risk factors. We know people that are older are more likely to get sepsis. People who have things like diabetes or who are immunosuppressed are more likely to get it. And different sites of infection are higher risks—pneumonia gives you a higher risk for having sepsis. You can get sepsis from a urinary tract infection, but it’s a much lower incidence. In general, younger people have a more robust inflammatory response, but it’s usually well-controlled. If it’s not, then younger people will often get this sick inflammatory kind of response and be really sick. And if you get pneumonia in your lungs, but it spreads into the blood, you are more likely to get this inflammatory response and sepsis.”

It seemed to happen so fast in Kyle Busch’s case. Is that common?

“There’s probably a couple potential things that happened. Even if you don’t have sepsis, one of the effects of pneumonia is it can make your oxygen levels low. So did he have low oxygen levels for a couple of days before he had this collapse and was taken to the hospital? Low oxygen, even without the sepsis, can cause damage to other organs. If they don’t get enough oxygen, they could start failing. A second possibility is an effect of sepsis called septic shock that makes your blood pressure low. When you have low blood pressure, you don’t get enough blood flow to your organs and then your organs start to fail. So did he collapse because his blood pressure was low from having an infection? And then subsequently lots of things started not working very well because his blood pressure was low?”

The day before he died, Busch was using a racing simulator. And the day after he died, he was scheduled to race. How is it possible he didn’t know how sick he was?

“Young healthy people tend to tolerate infection pretty well. But when it gets really severe and they get really sick, we call it falling off the cliff. They are cruising along, maybe they don’t feel very well, but they’re doing okay. And then suddenly everything fails and they are in the ICU really sick and their blood pressure’s low and we can’t get it up and their kidneys don’t work and their lungs don’t work and they’re on a ventilator. I’m guessing a little bit, but my bet is that he hadn’t felt well for a few days but he was in general doing okay and his body was kind of compensating. And then he kind of got to that point where his body couldn’t compensate anymore, and everything kind of fell apart at once.”

What else should people know about the risks of pneumonia and sepsis?

“If you’re having a fever and it’s lasting for a couple of days, if you have shortness of breath, if you have chest pain, you probably need to have a doctor or a healthcare provider look at you, and not just say, ‘If I just kind of ignore it’ll go away.’ With sepsis, it is a common thing and people just don’t know about it. Sepsis is a lot more common than people recognize. There are cases of teenagers that got sepsis who died, or young people that were otherwise doing fine and then they somehow got an infection and then next thing you know, they died. Young people like to think they’re immune to a lot of these things, but sepsis can happen to anybody. Young people do better than older people, but that doesn’t guarantee that you won’t die from it like Kyle Busch did.”