The Olympic gold medal-winning boxer was diagnosed with Parkinson's disease in 1984.
He was hospitalized twice in one month in late 2014-early 2015 for what was thought to be a mild case of pneumonia but was later determined to be a "severe" urinary-tract infection, according to the Associated Press.
A source close to Ali tells ET that he has been battling this respiratory issue for "some time." "He was invited to the Prince tribute held in Los Angeles a couple weeks back but could not make it because of this very issue," we're told.
Meanwhile, another source reveals that Ali is "getting weaker" and it is "harder for him to recover from chronic health issues."