TIME reported that Ephron had been diagnosed with the leukemia in 2006, but had not been public with her illness.
Ephron was behind iconic films such as "You've Got Mail," "When Harry Met Sally," "Sleepless in Seattle" and "Julie & Julia."
CBS News reported that Dr. Gail Roboz, who is Ephron's oncologist, said she died from acute myeloid leukemia. She passed away at Weill-Cornell/New York-Presbyterian Hospital.
However, the International Business Times reported that a Washington Post columnist was told that she actually died from the blood disorder myelodysplasia, which she had been diagnosed with years earlier.
Leukemia is cancer that occurs in the blood-forming tissues, such as the bone marrow, according to the Mayo Clinic. In people with leukemia, blood cells aren't produced properly by the bone marrow; over time, these abnormal cells grow and overcome the healthy cells.
There are expected to be 47,150 new cases of the disease this year, and 23,540 deaths, according to the National Cancer Institute.
The main types of leukemia are grouped into four kinds: two chronic, and two acute, each depending on what kind of cell is affected -- either the lymphoid cells, or the myeloid cells. Acute leukemia is aggressive and requires quick treatment, according to the University of Maryland Medical Center, while chronic leukemia occurs more slowly and can go undiagnosed for some time.
Ephron was reported to have acute myeloid leukemia. The Leukemia & Lymphoma Society said that treatment is imperative immediately after diagnosis with this kind of leukemia.
Risk factors for leukemia include genetic diseases, blood disorders, having been exposed to radiation or certain chemicals, having gone through cancer treatment previously, smoking, and having a family history of the disease, according to the Mayo Clinic.
The disease is treated with chemotherapy, radiation therapy, stem cell transplants to replace disease bone marrow with healthy bone marrow, biological therapy to boost the immune system and targeted therapy to attack the cancer cells, the Mayo Clinic reported.
Ephron was also reported to have died specifically of pneumonia as a complication of leukemia, TIME reported. Pneumonia occurs when the lungs become infected by bacteria or viruses that spread from other parts of the body to the lungs, when you breathe in bacteria or viruses, or when you breathe in fluids, food, liquids or vomit into the lungs, according to the National Institutes of Health.
Pneumonia can become deadly among older people, young children, people with lowered immune systems, or people who are already battling another medical problem, according to the American Lung Association. Complications include emphysema, sepsis (which can cause organ failure), and respiratory failure.
Note: This post was updated at 11:24 a.m., June 27.