There are about 6,000 people with haemophilia in the UK, most of whom are males because of the way the condition is inherited. Haemophilia is a bleeding disorder usually caused by a genetic defect that leads to a deficiency of clotting factor proteins in the blood. This shortage of clotting factors, which affects the blood's ability to clot, means that someone with the condition bleeds for longer than usual. The symptoms of haemophilia vary from mild to moderate to severe, depending on the level of clotting factors the individual has. People with severe haemophilia usually bleed frequently (once or twice a week), often spontaneously, into their muscles or joints. People with moderate haemophilia bleed less often, usually after an injury, but can also bleed spontaneously. People with mild haemophilia usually bleed only because of surgery or major injury.
The two most common types of haemophilia are haemophilia A (shortage of clotting factor VIII) and haemophilia B (shortage of clotting factor IX), and most people have haemophilia A. Both types have the same symptoms, even though they're caused by problems with different clotting factors and have slightly different treatments.
A rarer form of haemophilia, called acquired haemophilia, is not an inherited condition, but is caused by the immune system attacking and destroying the clotting factors in the blood, because the person has developed antibodies to factor VIII or factor IX. This usually occurs in middle-aged or elderly people, or young women who have recently given birth or are in the later stages of pregnancy. This condition is often resolved with appropriate treatment.
There are several other factor deficiencies that also cause abnormal bleeding. These include deficiencies in factors I, II, V, VII, X, XI, XIII and von Willebrand factor. The most severe forms of these deficiencies are even rarer than haemophilia A and B.