What is the deadly disease that has afflicted European royalty?

The deadly disease that famously afflicted European royalty was hemophilia. It is an inherited bleeding disorder in which the blood does not clot normally, so injuries, surgery, or internal bleeding can become life-threatening.

Hemophilia became associated with royalty because Queen Victoria of the United Kingdom carried a mutation for hemophilia B, a deficiency involving clotting factor IX. She passed the mutation through her daughters into the royal families of Russia, Spain, and Germany. Her son Leopold also had the condition.

The most famous case was Tsarevich Alexei, son of Russia's Nicholas II and Alexandra. His illness helped bring Grigori Rasputin to the imperial court, because the royal family believed Rasputin could ease Alexei's bleeding episodes. Genetic testing later identified Alexei's condition as hemophilia B.

Hemophilia is usually inherited through the X chromosome, which historically made males more likely to show symptoms while women could carry the mutation without severe disease. It is sometimes called the “royal disease,” although it has affected people in every social class.