Hemophilia is a disease in which a cut will not stop bleeding because blood will not clot. It is caused by a faulty gene that affects only men.
However, women act as carries of the gene, and they pass it on to their children. Queen Victoria of England inherited this faulty gene from her ancestor. She did not get the disease, but acted as a carrier.
She passed the disease on to Leopold, her fourth son. Her daughter who married of Europe did not get the disease either, but acted as carriers, and passed it on to their sons.
In fact, hemophilia because known as a royal disease, because of the number of descendents of Queen Victoria who where affected by it.