It has long been known that there is a very real condition where the sufferers grow extreme facial and body hair and have photophobia(literally - fear of light). This, merged with the legends of Vlad the Impaler have reinforced our images of the Vampire. A night dwelling creature that lives on blood drawn from live (or dead) hosts.
The vampiric ability (in films & books) is infective. Someone bitten by a vampire becomes one themselves, usually within a few hours. Large dogs can become vampires as well, but not small creatures like cats and toy dogs.
Is there a logical way of explaining all of this? Possibly.
The ideal method would be to have a bacterial species that infects the host. As a parasite.
In the 'real' world a bacterium called Agrobacterium tumefaciens, sometimes referred to as 'natures genetic engineer' modifies the behaviour of its host by inserting genes into the host. This causes the host plant to develop a tumour.
Viruses work in more or less the same way, except they can sometimes make mistakes when removing their DNA leading to genetic transfer between organisms which may account for the acceleration of evolution.
Parasites are usually extremely specialised - this bacterium would be specialised in digesting blood - specifically red blood cells. This would rapidly give the victim a very pasty appearance. But, bacteria replicate very rapidly under the right conditions, so the host would quickly die. If a small animal like a cat became infected it would probably die before being able to suck blood because the rapidly replicating bacterium would quickly digest so much blood that the host would not live long.
So the bacterium transforms host cells (by inserting the DNA). This could also interfere with the host immune response. Unfortunately it inserts the DNA inside the genes responsible for meat digestion. This disables the production of meat digesting enzymes. This is good news for the bacterium because the host now cannot digest meat except for blood. So the host now has to go blood sucking (or go and rob the Blood Bank). This ensures that the bacteria will spread.
However, this transformation can cause cells to become cancerous. [Some animal cancers are caused this way]. So the host might only have between 2 and 20 years to live. This won't bother the bacteria because it will have spread by then.
As the person sucks the blood of the victim( the ideal source being the jugular veins - arterial flow being too fast for a person to drink ) the bacteria will infect the 'donor. Only 1-2 pints need to be taken to keep the vampire alive. (for a while). This method
