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(CNN) —  

When ESPN reporter Edward Aschoff died, he had been diagnosed with multifocal pneumonia and a rare disease known as HLH, his fiancée tweeted.

Aschoff was first admitted to the hospital and diagnosed with pneumonia in many parts of his lungs but was brought back to the emergency room when antibiotic treatment failed and he got worse, Katy Berteau said.

“After many tests - bone marrow and lung biopsies - treatment was started for a presumed diagnosis of HLH,” she tweeted. “Within 3 days of being moved into the ICU, he passed.”

HLH, hemophagocytic lymphohistiocytosis, is a rare disease that affects the immune system.

She did not provide any further details about the manner of Aschoff’s death, which occurred on his 34th birthday.

Other people, including Aschoff himself, expressed surprise about the seriousness of the illness in a young man in apparently good health.

“Anyone ever had multifocal (bilateral) pneumonia in their early 30s as some who never gets sick and has a very good immune system? Asking for two friends … my lungs,” he tweeted on December 5.

More questions have come up about his second diagnosis, HLH. It is unclear if Aschoff had HLH or pneumonia first, if one came from the other, and exactly how he died so quickly.

Here is what we know about the diseases Aschoff’s had:

Is pneumonia dangerous?

Pneumonia is when air sacs in the lungs fill with fluid or pus. It can be caused by a virus, bacteria or a fungus, causing a fever and respiratory problems.

It can occur in one or both lungs, and multifocal means the pneumonia occurs in multiple places.

Thousands of people die around the world each year of pneumonia, but most healthy people can fight it off, especially with antibiotics and antiviral medications. The people most at risk are the young, elderly, frail or immune-compromised.

What is HLH?

HLH is a rare disease that affects the immune system, making certain white blood cells attack other blood cells and enlarging the spleen and liver, according to Johns Hopkins Medicine.

It can be inherited or acquired, Johns Hopkins said. About a quarter of cases are passed down through families, and the rest come from infections, a weakened immune system and cancer.

Symptoms can include coughing, difficulty breathing, fever, headaches, rashes, swollen lymph nodes, jaundice and digestive problems, according to Johns Hopkins.

Is it dangerous?

There is treatment for HLH, and acquired forms may clear when properly treated, Johns Hopkins said. If familial HLH goes untreated, it is usually fatal.

Treatments include chemotherapy, immunotherapy, steroids, antibiotic drugs and antiviral drugs. Stem cell transplants can cure HLH in most cases if drug treatments don’t work, Johns Hopkins said.

There is no way to prevent HLH, the medical center said.