ATLANTA — How does a healthy 34-year-old die after being diagnosed with pneumonia?

That's the question at the heart of the tragic and sudden death of an ESPN reporter earlier this week. 

According to college football reporter Edward Aschoff's fiancée, Katy Berteau, about a week after being diagnosed with a severe form of pneumonia, he began treatment for a "presumed diagnosed" of hemophagocytic lymphohistiocytosis (HLH). Three days after being moved to the ICU, Berteau tweeted from Aschoff's account on Thursday night, he died.

RELATED: Edward Aschoff's fiancée releases new information about ESPN reporter's sudden death, memorable life

HLH is a rare and complicated condition that's not entirely well-understood by researchers, but it basically stems from your immune system severely overreacting to an infection (such as pneumonia) or another illness. 

"It just gets overworked and starts fighting regular tissues, so usually those tissues are organs, like your spleen or your liver," Atlanta area doctor Will Epps said. "So it can result in liver failure or organ failure and unfortunately end up expiring from it."

Here's a general outline:

HLH and how it works

According to the Genetic and Rare Diseases Information Center at the National Institutes of Health, HLH comes in two forms: A genetic form and an acquired form.

St. Jude Children's Research Hospital says the genetic form is more common in young children, while the acquired form, sometimes called the secondary form, usually affects older children and adults, such as Aschoff.

What happens, broadly, is this: Faced with a severe infection, such as pneumonia (or other severe conditions like cancer), a person's immune system overreacts and, as the Johns Hopkins School of Medicine describes it, certain while blood cells called histiocytes and lymphocytes "attack your other blood cells."

"These abnormal blood cells collect in your spleen and liver, causing these organs to enlarge," Johns Hopkins says. 

(According to the NIH, HLH may also be associated with a separate genetic condition X-linked lymphoproliferative disease - XLP - when it results from an inappropriate immune response.)

It can cause death in a matter of weeks, according to researchers.

A 2012 paper in the medical journal "Clinical Advances in Hematology & Oncology," outlines adult HLH extensively.

"It is useful to think of HLH as the severe end of a spectrum of hyperinflammatory diseases in which the immune system causes damage to host tissues," the paper's authors, Dr. Roman Leonid Kleynberg and Dr Gary J. Schiller, wrote.

That paper estimated HLH occurs in only 1.2 cases per one million individuals every year, making it extremely rare.

Specifically amongst children, St. Jude estimates HLH is diagnosed in fewer than 1 out of every 50,000 - 100,000 children per year.  

The condition's mortality rate is difficult to pin down because it can fluctuate based on what caused it. Many sources say a common cause is Epstein-Barr virus, for example, and the 2012 paper reported the mortality rate associated with that 18-24 percent. Other causes can have lower mortality rates.

Generally, doctors try both to treat the underlying trigger for HLH and address the immune response.

"Treating that is tamping that down, either through steroids or with a chemotherapeutic agent which tends to attack or lower that immune system," Epps said.

John Hopkins Medicine also details antibiotic and antiviral drugs being used, or if drug treatment fails doctors may turn to stem cell transplants.

There is no known way to prevent HLH.

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