Lycanthropy is an illness which creates unnatural hair growth, like a werewolf, and their are people who have eaten human organs and drank blood due to physical and mental illnesses. These, however, are extremely rare.
Traditionally, history is written by the winners, and they justify acts of aggression by claiming that their opponents were barbaric and evil. The Romans claimed that the Carthaginians ate babies, as a justification for the Punic Wars, according to Tacitus and Cicero.
The truth regarding Vampires (in particular) is that the Roman Catholic church wanted to portray non Roman Catholic Christians as Devil Worshippers, hence their association with Romany Gypsies, whose faith comes from Jesus' time and have been persecuted by the RC church for millenia.
If you visit www.mylot.com, you will find two articles I have written (same user name) about Gothic novels and European literature by non Catholic authors about the Romanies and Vampires etc., notably Bram Stokers 'Dracula' and Victor Hugos 'Hunchback of Notre Dame'. You may find them interesting and informative.