Haemophilia (also spelled as hemophilia, from the Greek haima "blood" and philia "to love") is a group of hereditary genetic disorders that impair the body's ability to control blood clotting or coagulation. In its most common form, Hemophilia A, clotting factor VIII is absent. In Haemophilia B, factor IX is deficient. Hemophilia A occurs in about 1 in 5,000–10,000 male births , while Hemophilia B occurs at about 1 in about 20,000–34,000.

The effects of this sex-linked, X chromosome disorder are manifested almost entirely in males, although the gene for the disorder is inherited from the mother. Females have two X chromosomes while males have only one, lacking a 'back up' copy for the defective gene. Females are therefore almost exclusively carriers of the disorder, and may have inherited it from either their mother or father. In about 30% of cases of Hemophilia B, however, there is no family history of the disorder and the condition is the result of a spontaneous gene mutation. A mother who is a carrier has a 50% chance of passing the faulty X chromosome to her daughter, while an affected father will always pass on the affected gene to his daughters. A son cannot inherit the defective gene from his father.

These genetic deficiencies may lower blood plasma clotting factor levels of coagulation factors needed for a normal clotting process. When a blood vessel is injured, a temporary scab does form, but the missing coagulation factors prevent fibrin formation which is necessary to maintain the blood clot. Thus a haemophiliac does not bleed more intensely than a normal person, but for a much longer amount of time. In severe haemophiliacs even a minor injury could result in blood loss lasting days, weeks, or not ever healing completely. The critical risk here is with normally small injuries which, due to missing factor VIII, take long times to heal. In areas such as the brain or inside joints this can be fatal or permanently debilitating.

The bleeding with external injury is normal, but incidence of late re-bleeding and internal bleeding is increased, especially into muscles, joints, or bleeding into closed spaces. Major complications include hemarthrosis, hemorrhage, gastrointestinal bleeding, and menorrhagia.

Causes

• Hemophilia A involves a lack of functional clotting Factor VIII. (This represents 90% of haemophilia cases.)
• Hemophilia B involves a lack of functional clotting Factor IX.
• Hemophilia C involves a lack of functional clotting Factor XI.
• Hypofibrinogenemia involves a lack of functional clotting Factor I. Because it is so rare, about 1 to 2 cases per million births, it has no definite treatment approved by the FDA. It affects males and females equally. The blood of people with Hypofibrinogenemia neither clots nor contains sufficient amounts of Fibrinogen.