Chuck Pagano is only the second head coach in recent NFL history to be diagnosed with cancer during the season, according to Indianapolis Colts Owner Jim Irsay.

Pagano was hospitalized Wednesday night and immediately began treatment after being diagnosed with "acute promyelocytic leukemia," a subtype of acute myeloid leukemia, "which is a cancer of the bone marrow tissue," according to his physician Dr. Larry Cripe, a leukemia expert from the Indiana University School of Medicine.

Here are a few things to know about leukemia and specifically acute promyelocytic leukemia or APL:

How common is APL?

Each year, more than 40,800 adults and 3,500 children are diagnosed with all kinds of blood and bone marrow cancer called leukemia, according to the National Cancer Institute. About 13,780 of these new cases will be adults diagnosed with acute myeloid leukemia (AML).

APL - the cancer Pagano has been diagnosed with - is a rare subtype of AML. Only 10% of AML cases, or about 1,300 people, are diagnosed with APL each year.

"Everyone who is diagnosed with acute myeloid leukemia faces a dangerous situation," Cripe told reporters Monday.

What are the symptoms?

Irsay told reporters Monday that Pagano was suffering from fatigue and unexplained bruising, and at the urging of his wife, Tina, the coach took advantage of a week with no game to go to the doctor.

Those are two common symptoms for leukemia, says Dr. Harry Erba, director of hematologic malignancies at the University of Alabama at Birmingham Cancer Center.