Haemophilia

Haemophilia, also called the royal disease, was a hereditary human disease that stopped the blood from clotting.

Queen Victoria passed haemophilia to many of her direct descendants, including Alex Romanov, the heir to the Russian throne. (PROSE: The Clockwise Man) According to the Tenth Doctor, it was always a mystery, as Victoria did not inherit it.

He hypothesised that this illness was in fact a cover, a Victorian euphemism, for having been infected by a Lupine Wavelength Haemovariform, and that Victoria's descendants in the Royal Family by the 21st century were werewolves. At first incredulous, Rose Tyler thought of Princess Anne, as well as how they were very private, were able to schedule themselves around the moon, and loved hunting, and agreed with the Doctor's hypothesis. (TV: Tooth and Claw)