I just solved the mystery of vampires!
Their skin has AgCl which decomposes to form Cl2 in the presence of sunlight. Cl2 is toxic to humans as it way cause the release of free oxygen in the cells amoung other things...
They suck people's blood because they can't get vitamin D
They are allergic to garlic because AgCl works as a catalyst for the garlic enzymes and it can dissolve their skins.