Risk factors include smoking and alcohol consumption.
Kelly's diagnosis stems from pain he began experiencing in his jaw in December. He initially thought it was an infection, but grew concerned when antibiotics failed to help.
Tests eventually led to doctors removing a nickel-sized cyst from his gums and nasal cavity during an operation in early March. Follow-up tests revealed the cancer.
It's the latest operation Kelly has required over the past two years. He's also had surgery to correct back, neck and hernia problems.
Inducted into the Pro Football Hall of Fame in 2002, Kelly still holds nearly every significant career Buffalo Bills passing record: 35,467 yards, 237 touchdowns and 26 300-yard games.
Kelly spent 11 seasons with the Bills before retiring following the 1996 season, and has since made Buffalo his home. Known for his fearless, swashbuckling style, Kelly was the face of Bills teams that made four consecutive Super Bowl appearances in the early 1990s, only to lose them all.
Kelly intended to draw upon his faith and family, and the perseverance he's developed in facing other challenges in his life both on and off the field. Kelly's son, Hunter, was born with Krabbe disease, an inherited degenerative disorder of the central and peripheral nervous systems. Given little more than three years to live, Hunter died at the age of 8 in 2005.