Hemophilia is a hereditary blood disease characterized by the inability of blood to clot, or coagulate, leading to hemorrhage , or excessive bleeding, even from minor injuries. The disease is caused by an insufficiency or absence of certain blood proteins , called factors, which participate in blood clotting. The most common form, hemophilia A, is observed in 80 percent of hemophiliacs and is caused by a lack of factor VIII; in the second most common, hemophilia B (Christmas disease), factor IX is missing. The severity of hemophilia varies greatly. The bleeding may occur as excessive bruising or persistent bleeding after a simple cut. Hemorrhaging into joints and muscles can be disabling. Before the advent of modern therapy, the chance of survival to adulthood was poor (caption above: This child has a severe swelling of the forearm from a bump to the elbow. This was caused by a failure of his clotting system to stop the bleeding from a bruise. Hemophilia can be controlled with the infusion of factor VIII collected from donated blood or plasma )