Hemophilia is a hereditary disorder that prevents a person’s blood from clotting properly.  It is rare and occurs almost exclusively in males. About 18,000 Americans have hemophilia, according to the U.S. Centers for Disease Control and Prevention (CDC).

Normally, a person who suffers an injury to a blood vessel experiences a chain of events that causes the blood to clot and seal off the wound. After an injury to a blood vessel, small blood cells called platelets stick to the vessel at the site of the injury. Proteins called clotting factors help the platelets to stick together with calcium and other tissue factors in a clump known as a clot. This helps plug breaks in blood vessels, causing bleeding to stop. Human blood has 20 clotting factors that contribute to this process. After the wound heals over time, the clot dissolves.

People with hemophilia lack one of two clotting factors essential to preventing excess bleeding. This makes them vulnerable to excess bleeding when they are injured. The loss of excessive amounts of blood due to an external wound can damage internal organs. Left untreated, internal bleeding caused by hemophilia can damage joints, which can limit mobility.  Internal bleeding into a vital organ such as the brain or heart can be fatal.

Hemophilia can be classified as mild, moderate or severe. The level of severity is determined by the amount of the specific clotting factors an individual has in their blood. For example, a person with severe hemophilia A has less than 1 percent activity of factor VIII. By comparison, people without hemophilia have 100 percent factor VIII activity.

The amount of bleeding that patients experience varies from individual to individual. For example, some patients with mild hemophilia may not bleed often, while some patients with severe hemophilia may bleed frequently.

In addition, some patients develop a rare, noninherited form of hemophilia when certain antibodies form and interfere with the functioning of clotting factor VIII or clotting factor IX. This is known as acquired hemophilia.

There are three types of inherited hemophilia:

• Hemophilia A (also known as classic hemophilia or factor VIII deficiency). Clotting factor VIII is low or missing in the patient. About 90 percent of people with hemophilia have this type. About 70 percent of the patients with this type of hemophilia have severe hemophilia.
  
  
• Hemophilia B (also known as Christmas disease or factor IX deficiency). Clotting factor IX is low or missing in the patient. About 10 percent of people with hemophilia have this version of the disorder. Christmas disease refers to the first patient with hemophilia B who was studied in detail, Stephen Christmas.
  
  
• Hemophilia C. Clotting factor XI is low or missing in the patient. This type of hemophilia is extremely rare in the United States and symptoms are typically mild.

The manner in which the medical community deals with hemophilia has experienced several radical changes during medical developments of the 20^th century. For most of history, people who had it frequently died or were disabled by joint- or organ-damaging bleeding. Today, the prognosis for people with hemophilia has vastly improved. The identification of the clotting factors in the 1960s and the ability to treat people with clotting factors drawn from donated blood led to longer, healthier lives.

However, in the late 1970s and early 1980s, many people with hemophilia were infected with viruses that were transmitted through blood donations, including hepatitis and HIV, the virus that causes AIDS. In the early 1980s, about half of the people with hemophilia contracted HIV through blood products and many of them died, according to the National Hemophilia Foundation. Blood screening for these viruses began in 1985, which has essentially stopped this form of transmission. In addition, a newer kind of clotting factor became available in 1992, when researchers were able to synthesize the product in laboratory proteins, without the need of human blood. The resulting recombinant products cannot transmit viruses, although it is still uncertain whether other disease-causing pathogens can be transmitted through these products.