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.

ESPN said Aschoff died on Tuesday after a brief illness, without elaborating on a cause of death. Aschoff tweeted on Dec. 5 that he had pneumonia.

While often thought of as an illness that affects the sick and the elderly, and while older adults, people with weakened immune systems and young children are more at risk, pneumonia can strike indiscriminately, said Dr. Humberto Choi, a pulmonologist at Cleveland Clinic in Ohio. (Choi was not involved with Aschoff's case.)

"This is an example that anyone can get pneumonia, and it can be severe, even when that person is in good health," Choi told NBC News.

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.

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.

“In an otherwise healthy young adult, barring anything unforeseen, treatment can be fairly straightforward and patients can recover within a few weeks," Azar 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.