Hemophilia is a genetic disorder caused by a mutation of clotting factor genes.  This disease, the oldest known hereditary bleeding disorder, affects mostly males (though it can be active in females).  Despite this, only one in every 4,000 men contracts the disease.  Hemophilia is fairly rare, so rare in fact, that there are only approximately 20,000 recorded cases in the United States and only 400 babies born with the disorder each year.  Of these recorded cases, 70% have severe hemophilia.