Joe Biden's prostate cancer diagnosis has come after earlier intense public scrutiny of his health fitness for the presidency. 

The former US President has an aggressive form of prostate cancer, with possible treatment available, although it's not curable and has spread to his bones. 

US correspondent Jonathan Kearsly says it comes at an interesting time - when his legacy is being questioned. 

"Hard questions, obviously, over his health, his mental acuity while he was in office - and now, two days before a book is set to be released about those very concerns."

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