UPDATE:  Bill Paxton's death certificate states the actor died last month from a stroke days after surgery to replace a heart valve and repair damage to his aorta, the Associated Press wrote on Monday. 

Details are scarce, but in an otherwise healthy man of that age, the risk of stroke during heart-valve surgery is less than 1 percent, according to a risk calculator from the Society for Thoracic Surgeons.

Paxton, whose death was announced Sunday in a statement from family representatives, said earlier this year that he had suffered valve damage from a case of rheumatic fever as a child.

Rheumatic fever can damage the heart's mitral valve, and less commonly the aortic valve, said Kimberly Campbell, a partner with Cardiology Consultants of Philadelphia.

This type of fever has become less common due to antibiotic therapy that is administered for strep throat, Campbell said. If strep isn't properly treated, it can lead to rheumatic fever.