Rheumatic fever was the single biggest cause of valve disease before widespread availability of good antibiotics.
Most valvular disease in the United States at the current time is due to:
Myxomatous degeneration: Age-related degeneration - It happens most often in elderly patients and commonly affects the mitral valve.
Calcific degeneration: A buildup of calcium on the aortic or mitral valves due to age or other reasons causes thickened valves which prevents them from closing well.
Congenital (genetic) disease: Irregularly-shaped valves that people are born with.
Weight loss medicines such as fen-phen and Redux: These can accelerate valvular damage.
Endocarditis: An infection in the lining of the heart's walls and valves. Sometimes infections can destroy the valves.
Coronary artery disease: This can cause functional limitations in the parts of the heart muscle which support the heart valves.
Hypertension: Can lead to degeneration (wear and tear injuries).