Do I believe there are beings that can turn into bats, get harmed when touched by a cross, and burn into ash in sunlight? No.
Do I think there could be a disease--or could have been in times past--that gave people an ashen pallor, hypersensitivity to light, allergic-type reactions to substances such as garlic, and an urge to drink blood? Well, that seems more feasible, but I don't think there's enough evidence for it.
I read a short story once that attempted to explain vampirism as an illness and medically explained the symptoms. (Or as medically as possible, since the story took place during the plague outbreak in London in the 1600s.) There was nothing about bats or crosses or anything, that I can recall. The story is what planted the seed of that 'disease' idea in my head. But I still think it mainly just makes for good story ideas--I doubt there has ever really been a disease like that.