Different diseases can affect each valve. Diseased valves may either become blocked, preventing adequate blood flow due to calcifications or scar tissue, or they may become leaky, allowing blood to flow backwards. In some cases, the defective valve may suffer from both problems: the edges of the valve can thicken so its leaflets no longer fit together properly, preventing an adequate forward flow while still allowing some reverse leakage. The most common valvular diseases among adults are aortic stenosis and mitral valve prolapse.