Muhammad Ali in hospital: How serious is his condition?

Muhammad Ali has been admitted to hospital after experiencing "respiratory issues".

"He is being treated by his team of doctors and is in fair condition," family spokesman Bob Gunnell said in a statement. "A brief hospital stay is expected. At this time, the Muhammad Ali family respectfully requests privacy."

The statement did not give details of which hospital is treating the former heavyweight champion, but ESPN notes the missive is datelined from Phoenix, Arizona, where Ali lives.

The 74-year-old's current hospital stay follows similar spells under doctors' care in 2014 and early 2015, initially thought to be for pneumonia but later identified as a severe urinary tract infection.

However, the Associated Press cites an unnamed source claiming that Ali's current condition may be more serious than the statement suggests. Two people, who asked not to be identified, told AP the respiratory issues were complicated by the former boxer's three-decade battle with Parkinson's disease

Ali was diagnosed with the illness in 1984. Doctors believe it is linked to the many blows to the head he withstood during his lengthy career in the ring. Most of his increasingly rare scheduled appearances are for benefits and fundraisers connected to the disease.

Visibly frail, Ali no longer speaks in public and lives a private life with his fourth wife, Lonnie.