In modern fiction the assumed way to become a vampire is to be bitten by one. La Fanu used the old legend that vampirism is the result and curse from suicide.(7) When she was human Carmilla attempted to get attention from her family that she felt ignored her by trying to kill herself. Unfortunately the attempt went too far and she was cursed to come back and feed of the blood of the living. This was actually a pretty common belief of the cause of vampires prior to Dracula.(8)
The animal that modern vampires are most associated with is, of course, the bat, though it was stated that Dracula could turn into a dog. Carmilla’s animal guise was that of a large cat. Looking much like a North American Puma it is in this form that she travels the country side looking for victims. .
Carmilla also differs from later vampires in the fact that she can be outside in the sun. She is best described as nocturnal. She is week and languid during the day time, and her skin reacts badly to the daylight, but she does not explode into flames during when being exposed to day light.
I have two theories about Carmilla that I have not seen elsewhere in my research on the subject. (Though admittedly my internet wandering are far from scholarly, academic or thorough) In 1858 La Fanu’s wife had what was probably a nervous breakdown that lead to her declining health and eventually lead to her death.(10) In an attempt to make sense of a world that seemed to be closing in on her, before her demise, she attempted to obtain solace in the doctrine of Church of Ireland, her more and more fanatical beliefs lead La Fanu to accept atheism. To me it seems like that Carmilla’s strange sometimes irrational behavior and her hatred of God may have its genesis in this period of the writer’s life. We have defiantly seen writers like Philip K. Dick, Lovecraft and Jules Vern take inspiration from real life tragedy and it is not much of leap of faith to assume that La Fanu may have done the same.