Normally inherited, hemophilia is a rare bleeding disorder rendering blood unable to clot normally. As a result, even the smallest of childhood -- or adulthood -- bumps, bruises, and scrapes can be quite serious and possibly life-threatening.
Most of us are born with proteins, or clotting factors, that interact with platelets, enabling them to clot to stop bleeding. If proteins are absent, or limited, the platelets are left unable to do their job properly and bleeding -- even from minor cuts and scrapes -- may continue unchecked. Along with free bleeding from cuts and scrapes, those with hemophilia run a greater risk of internal injury and bleed, causing damage to joints as well as internal organs.
The two most common forms of hemophilia are hemophilia A and hemophilia B. About 90 percent of all persons with hemophilia have hemophilia A. Persons with hemophilia A lack normal levels of clotting factor VIII.
https://www.empowher.com/hemophilia/content/hemophilia-overview