The news that ESPN reporter Ed Aschoff died on his 34th birthday after being treated for pneumonia is a reminder, experts said, of how deadly the infection can be, even in otherwise healthy young people.

Aschoff's fiancée, Katy Berteau, wrote on his Twitter account Thursday that Aschoff was first diagnosed with pneumonia but was then treated for a presumed diagnosis of HLH, or hemophagocytic lymphohistiocytosis, which is an immune system disease.

(6/12) After many tests - bone marrow and lung biopsies - treatment was started for a presumed diagnosis of HLH, an unregulated, over-activation of the immune system that causes it to attack itself and other healthy tissues. Within 3 days of being moved into the ICU, he passed. pic.twitter.com/O8v3eZqefX — Edward Aschoff (@AschoffESPN) December 27, 2019

Pneumonia can strike indiscriminately, said Dr. Humberto Choi, a pulmonologist at Cleveland Clinic in Ohio. (Choi was not involved with Aschoff's case.) However, older adults, people with weakened immune systems and young children are more at risk, he said.

Ed Aschoff. Rich Arden / ESPN Images via AP

HLH could cause a person's immune system to be weakened, putting that person at higher risk for complications from pneumonia, Choi said.

But HLH can also be triggered by an infection like pneumonia. In adults, it's more common for HLH to be triggered by an infection or another condition, rather than the other way around, Choi told NBC News. Even if a person a genetic carrier for HLH, they may not know they have it until something triggers the acute condition.

HLH is very rare and can be life-threatening. The disease affects multiple organs throughout the body, and can look similar to sepsis, another system-wide condition, he said. This makes it difficult to diagnosis.

Berteau, Aschoff's fiancée, wrote on Aschoff’s Twitter account Thursday in a series of tweets that Aschoff was initially diagnosed with multifocal pneumonia, but the antibiotic treatment did not work, and he was then treated for a presumed diagnosis of HLH, which is "an unregulated, over-activation of the immune system that causes it to attack itself and other healthy tissues."

She said that within three days of being admitted to a hospital’s intensive care unit, he died. Berteau thanked people for an outpouring of love and admiration for Aschoff’s life, as well as for their prayers and condolences.

"I want to share the brightness that he showed, even up until the last day he was awake. He kept the doctors and nurses constantly laughing, and always made a point to thank them and tell them what a great job they were doing," she wrote.

Pneumonia is serious

Experts told NBC News that pneumonia is a serious illness, and can affect even seemingly healthy people.

"Anyone can get pneumonia, and it can be severe, even when that person is in good health," Choi said.

Pneumonia is an infection that causes inflammation in the air sacs of the lungs. These air sacs, called alveoli, can fill with fluid, making it difficult to breathe. The disease can be fatal, especially this time of year, when respiratory infections are more common, Choi said.

The flu, in particular, can increase a person's risk of developing pneumonia.

"This is why we talk so much about the flu shot," Choi said.

The flu can lead to pneumonia in several ways. The virus itself can cause viral pneumonia, but having the flu can also put at person at risk for bacterial pneumonia, which is considered more severe.

Some cases of viral pneumonia can clear up on their own, without treatment, said Dr. Natalie Azar, an assistant professor of medicine and rheumatology at NYU Langone Medical Center in New York City and an NBC News medical contributor. Bacterial pneumonia, however, must be treated with antibiotics.

It's unclear if Aschoff had bacterial pneumonia, but he said in his Dec. 5 tweet that he was taking antibiotics.

Damn. So I had a virus for two weeks. Fever and cough and the doctors think it turned into this multifocal pneumonia recently. I’m on day 4 of antibiotics. Days are getting better but nights are basically fever and coughing and sweating. How long were you out for? — Edward Aschoff (@AschoffESPN) December 5, 2019

Choi, who works in an intensive care unit, said that pneumonia is one of the leading causes of patients' needing intensive care. The inflammation can be overwhelming, he said, and patients may need oxygen and life support.

Not every case of pneumonia is severe, however, and many can be treated with outpatient care, Choi said.

"It must be remembered, however, that pneumonia is serious and can be life-threatening in certain situations," she said. "In certain cases, the infection just overwhelms someone's immune system."

In the U.S., more than 250,000 people are hospitalized each year from pneumonia, and about 50,000 people die, according to the Centers for Disease Control and Prevention.

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