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Deficiency in coagulation factor VIII is the most common cause of haemophilia.
Haemophilia (also spelled hemophilia in North America, from the Greek haima αἷμα 'blood' and philia φιλος 'love') is a group of hereditary genetic disorders that impair the body's ability to control blood clotting or coagulation, which is used to stop bleeding when a blood vessel is broken. Haemophilia A (clotting factor VIII deficiency) is the most common form of the disorder, occurring at about 1 in 5,000–10,000 male births. Haemophilia B (factor IX deficiency) occurs at about 1 in about 20,000–34,000 male births.