Hemophilia is a disorder of the blood -clotting system. Normally, when you cut yourself your blood plugs the wound by changing from a liquid to a solid and forming a blood clot. If you have hemophilia, your blood will not clot normally and you may bleed for longer than normal, or you may bleed internally, particularly into joints such as your knees, elbows and ankles.

Blood contains special proteins, called 'clotting factors', which are involved in the clotting process. There are 20 different clotting factors. If you have hemophilia, your body does not produce enough, or produces none, of one of these clotting factors.

There are different types of hemophilia. If you have hemophilia A, you have a deficiency of clotting factor VIII (known as factor VIII). Hemophilia A is also known as classical hemophilia and is the cause of about 80% of cases. If you have hemophilia B, you have a deficiency of clotting factor IX (known as factor IX). Hemophilia B is also known as Christmas disease and is the cause of about 20% of cases. Deficiencies of factors VIII and IX are the most common; deficiencies of other factors do exist but are very rare.

Hemophilia can be mild, moderate or severe, depending on how much factor VIII or IX is missing from your blood:

• Mild hemophilia - your blood contains 5-25% per cent of the normal amount of factor VIII or IX.
• Moderate hemophilia - your blood contains 1-5% of the normal amount of factor VIII or IX.
• Severe hemophilia - your blood contains less than 1% of the normal amount of factor VIII or IX.

Hemophilia is rare and almost always affects men. Hemophilia A occurs in about 1 in 10,000. Hemophilia B occurs in about 1 in 40,000.