For the purpose of this question (to make the idea more plausible) a vampire is not an undead creature. Rather:
- A vampire is a human, infected by a symbiotic virus;
- The virus is present in a vampire's saliva, spreading infection whenever it has contact with an uninfected human's blood (a bitten human becomes a vampire);
- Perhaps a vampire's children are also vampires;
- An infected human, perhaps after a period of illness, develops:
- Increased ability to see in the dark, matching the best animals; However, also an increased eye vulnerability to intense light, to the point of having to squint eyes in lighting humans would consider "normal" and to the point of not being able to withstand the brightest hours of the noon;
- Irises would become red (a side effect of the above eye mutation);
- Maxillary canines would double their size (after some time of growth obviously); From this point on, all teeth, if broken or knocked out, would regrow slowly;
- Skin would have a noticeably paler shade (understandable, since a vampire would be a primarily nocturnal creature, as opposed to an uninfected human);
- A vampire would be able to digest blood; actually, a vampire would have to drink blood to survive (impaired body's ability to produce erythrocytes?)
- A vampire's skin would become vulnerable to silver. Contact with silver would cause possibly severe burns. This would create problems in regions where silver was an established currency and/or material for minting coins.
- They would also be allergic to garlic.
- Maybe: A vampire would be able to survive by drinking animals' blood as well, not necessarily uninfected humans' blood, and wouldn't feed an uncontrollable urge to bite uninfected humans. Maybe this would depend on the variation of the virus?
Ah, and vampires would age and die from age normally. (To make this less troublesome).
- How could such a virus evolve;
- Why would this virus evolve to be actually beneficial for its host, rather than only causing severe diseases; are symbiotic viruses known in the nature at all?
- A bitten human becomes a vampire; this implies an exponential rate of infection spreading (the more infected vampires there are, the more uninfected humans they can bite) - would there be any uninfected humans left after a few generations at all?
- Partially for this reason I said about the lack of an overwhelming desire nor need to bite humans; wouldn't the virus, however, likely create such a need to hasten its spreading? (Might be explained by spreading through inheritance though, it might be "enough" for the virus)
- Another possible explanation: The virus' variations which would necessitate drinking uninfected humans' blood or create an overwhelming urge to infect would die out either because, after a few generations, there would no longer be any uninfected humans alive whose blood could be drunk, or because humans would desperately fight and exterminate all infected individuals. Thus, less severe variations of the virus would be evolutionarily advantaged
- Would the transformation be biologically feasible?
- How could one explain the drawbacks, especially the vulnerability to silver and garlic? This seems arbitrary.
- Why maxillary canines, wouldn't central incisor work better?