Magihaematophagia (Proto-Vampirism) is a magia morbus, or magical disease.
This disease alters the digestive system of the afflicted so that they can only digest blood - where the blood is from does not matter, but anything else is rejected violently. This ends up depriving the body of other vital nutrients, and thus more often than not leads to the death of the afflicted.
It is believed that Vampirism evolved from a particular strain of this disease, hence its nickname "Proto-Vampirism".