Hemophilia is a serious genetic disease where blood in the body is unable to clot due to the lack of the factors that cause blood to clot. The disease has been known for centuries and was once endemic in the British Royal Family. It has now been traced to a defective gene on the X-chromosome (the female sex chromosome). As the defective gene is rare and recessive, it is rarely suffered by women as they usually have at least one good gene on one of their two X-chromosomes. However, in males, the Y-chromosome does not have the gene and a male with the gene inevitably develops the disease. Ironically, this means the son of a mother who carries the gene has a 50/50 chance of developing the disease, where the children of a male with the disease has no chance of developing it.
Hemophilia was untreatable until the middle of the 20th century, the prior prognosis being that the patient could live a normal life as long as they didn't suffer a major traumatic injury, which would inevitably lead to death from internal bleeding. However, once platelets could be isolated from blood, they could be injected into patients after injury to start clotting. However, by the 1970s, human clotting factor could be harvested from large numbers of donors, allowing patients to inject themselves at home rather than making a trip to the hospital.
However, during the early history of AIDS, many hemophiliacs became infected by the virus through clotting factor, which was much more likely to be infected than single donations would be. Although Great Britain destroyed its stocks of clotting factor that might be contaminated (and faced criticism from hemophiliacs for doing so), most other countries were not convinced at the time that AIDS was blood borne, or believed the virus could be destroyed by heating.