As the body naturally decomposes, you can actually explain some of the common “first signs” of vampirism.
New skin growth – The epidermis will shed off the corpse. Blisters will form and begin to burst, leaving reddened, “new” skin.
Bloating – This is a common sign of a vampire in Greece and with the Serbian gypsies- if a body bloated before burial, it would most likely become a vampire. But, bloating and swelling occur when the gasses that are produced within the intestines of a corpse bloat the body. The microorganisms that are formed during the decomposition process produce gas (mostly methane) throughout the tissues, and without a place to escape, the gas collects in the tissues and cavities of the body.
Blood by the nose and mouth – The blood that drains from the nostrils and the mouth can be explained as progressive disintegration and purification of the corpse. Blood coagulates after death, but depending on how the person died, the blood may remain coagulated or it may liquefy again.
“Fresh” blood around the mouth or still in the body when exhumed – The fresh blood can be explained by the liquefaction of the coagulated blood.
Pale complexion – After death, circulation of blood stops and the blood will fall to the lowest part of the body, leaving the face a pale color, as though it was drained of blood.
The body sat up – Corpses will suddenly sit up or move some time after death. The body will rise elegantly, without hesitation. There are certain chemicals and fluids within the throat and trachea that continue to function even after death. And, because the body has its own post-death system that functions to prepare the body for deterioration and decomposition. These fluids will contract after the rest of the bodily fluids dry up, which draws the other organs to contract. The muscles and tissues within the stomach and lower digestive tracts are reduced, and during this process, the body will arch forward into a sitting position.